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March 

1985 


©Dsietrontei 


> • EPROM switchboard 


• RLC meter • easy music 

* electronics the easiest way 






SONODYNE 

name that's music to your 


Sonodyne's in-depth experience in 
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performance, sleek styling, 
convenience and versatility. 


• Dolby is the registered trademark 
of Dolby Laboratories. 

Licensing Corporation. 


Introducing the Top -of-the- Line 
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Metal Tape Facility and Soft Touch Controls: 

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* 


Exceptional features at a mini price! 


Sonodyne's D-700 is its own spokesman. Switch it on at your nearest 
Sonodyne dealer. It won't take you long to find out it's the best 
value cassette deck in the market! 


Highlights of D-700: 

• Slow eject type cassette door and soft-touch push-button 
controls • Heavy duty Japanese tape deck mechanism to reduce 
wow and flutter • Dolby * noise reduction circuitry 
Tape selector facility for using Normal. Chromium Dioxide and 
tapes • Sendust head for metal compcsition tapes 
• Double LED VU meter 


Dattaram-SE 45 A/83 





Volume 3-Number 3 


EDITOR: SURENDRA IYER 
PUBLISHER: C R CHANDARANA 
PRODUCTION: C N MITHAGARI 

ADVERTISING & SUBSCRIPTIONS 
eIeI<TOR ElECTRONiCS pVT It(J. 

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Off Tulsi Pipe Road, 

Lower Parel, BOMBAY- 400 013. 


Elektor India is published monthly under 
agreement with Elektuur B.V. Holland. 
August/ September is a double issue. 


SUBSCRIPTION 

INLAND 

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FOREIGN 

One year Only 

Surface mail Rs. 125/- Air mail Rs. 210/- 


The Circuits are for domestic use only The 
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implies permission to the publishers to alter and 
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contents in other Elektor publications and 
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return any material submitted to them All 
drawings, photographs, printed circuit boards and 
articles published m Elektor India are copyright and 
may not be reproduced or imitated in whole or part 
without prior written permission of the publishers 

Patent protection may exist in respect of circuits, 
devices, components etc described in this 
magazine 

The publishers do not accept responsibility for 
failing to identify such patent or other protection 


INTERNATIONAL EDITIONS EDITOR: P HOLMES 


Dutch edition. 

German edition 
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French edition: 

Italian edition 
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Elektuur b v 
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the Netherlands 
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Portuguese edition Ferreira and Bento Lda 

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COPYRIGHT £ ELEKTUUR BV - 
THE NETHERLANDS 1984 



electronics the easiest way 3-12 

It is well known that child: en can generate some of the most original ideas on any subject 
We can only envy the straight forward simplicity of their answers to questions of a 
technical nature esoeciallv when the subiect is electronics 

news, views, people 3-15 

selektor 3-17 

2000 kW under the sea! 

gyroflash 3-20 

An inventive design comprising five xenon tubes flashing sequentially which has 
possible applications in photography, scale modelling, or as a distress light, to name 
but a few. 

1.2 GHz input stage 3-24 

Designed primarily for our recently published pP-controlled frequency meter, this 
wide band input stage can also be used with other designs. 

microphone preamplifier 3-30 

Dual circuit for improved signal-to-noise ratio, more effective hum suppression, and 
symmetrical signal transfer. 

remote model control by microcomputer 3-32 

A look at some of the new pcm equipment now becoming available from European 
and Japanese manufacturers. 

DIY connector 3-39 

The solution to the eternal problem of what to do when a project you are building 
needs a special connector, or simply one you do not have. The answer? Look in your 
junk box! 

programmable rhythm box 3-40 

Set your microcomputer to work on a useful (?) task: controlling an electronic drum 
box. The ZX81 is used as an example to show that even small computers can be 
used for control purposes. 

EPROM selector 3-46 

A simple way of expanding the memory of your microcomputer. 

easy music . . 3-49 

RLC meter 3-50 

A universal tester of resistance, capacitance, and inductance, which is simple to 
build, inexpensive, and yet quite accurate. The use of high-quality components 
makes an accuracy of 1 per cent possible. 

programmable keyboard encoder 3-56 

This versatile static 80 key matrix can be used for most applications where an alpha- 
numeric keyboard encoder is required. 

applicator 3-60 

Digital graphic equalizer based on National Semiconductor's LMC835. 

market 3-62 

switchboard 3-69 

missing link 3-72 

index of advertisers 3-74 


If 



elektor india march 1985 3.03 







COLOUR TV 
MANUFACTURCRS 

FOR YOUR REQUIREMENT OF: 


Oh MAI HiXfi AM) TOOL I ORROR tTIO.X 

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TELEX 232395 TELEX 125091 


AVAILABLE UNDER OGL APPENDIX No. 1 PART B 


BALAJI ENGINEERING CO. BALAJI ENTERPRISES, 


1 95, Brigade Road 
Bangalore-560 001 
Phone : 5241 1 


B-1 5, Prashanth Apartments, 

Macintyre Road, 

Secunderabad-500 003. Phone : 77490 


Mr. S.M. LIMDI 

Krishna Krupa 

10th Khetwadi Lane 

Bombay - 400 004 Phone 389457 

MYSORE: Phone 27737 


FIVBRCK 

TRANSFORMERS 


CONTACT: 


DANNIES ELECTRONICS ENTERPRISE 

Importers and Fxporters 
77. High Street # 10-12, High Street Plaza. 
Singapore 061 7. 

Phone: 3372297. 3397696 Telex: RS 28612 ESQIRE 


MEASUREMENTS 


6-200/ R 3278 


MANUAL WIRE-WRAPPING TOOLS 

CHUCK TYPE 

The G100 R3278 and G200 R3278 
loots are designed with a 
chuck nose piece to make use of the full 
line of bits and sleeves made for power 
tools These tools accommodate wire sizes 
from AWG22 (0.65 mm) thru AWG32 (0.20 
mm) and can provide a maximum of ten 
turns Fewer wraps can be achieved by 
adjusting the "strip'' length of the wire. For 
production and field service use in the 
electronic, telecommunications, and 
appliances industries. 

When wire-wrapping on 025" x 025" (0.63 
x 0.63mm) posts use maximum "strip 
length 1 ' of '/«' (22 mm). 

SEE CHARTS ON PAGES 70 AND 71 FOR 
PROPER SLEEVES AND BITS TO USE WITH 
THESE TOOLS ’ 


0 1 pf/uH/m otyn fie. 0.0001 ohm) to 20.000uf/200H/20 M ohm 

Vasavi Electronics 

(Marketing division) vnqqe 

630, Alkarim Trade Centre . Ranigunj 
SECUNDERABAD 500 003. gms: V 


Measurement of INDUCTANCE. CAPACITANCE. RESISTANCE are area 
simplified by VLCR 7. 


Connect the component to the terminals. VLCR 7 gives you directly 
the digital reading of value and its loss factor simultaneously. 

FOUR TERMINAL measurement elimigates inherent errors due to lead 
resistance. GUARD TERMINAL provided eliminates errors due to lead 
capacitance. 

VLCR 7 is the only instrument in India covering the widest ranges of 


3.04 elektor mdia march 1985 





TAPE HEAD 
.CLEANING. 


**• 78 * 

AtWOSOC 

CU»M*. ,k 


CO«H»'‘ 




poo -pSoglE- 

SOPHISTICATED ELECTRONIC 
CIRCUITS AND SYSTEMS DEMAND 
PERFECT MAINTENANCE ! 

-ONLY Mi lilt® TF-787 

CAN ENSURE IT 


Perfect maintenance means cleaning 
even the microscopic spots of grit, dirt, 
flux and oxide build-up that may affect 
the smooth functioning of the 
equipments. 

KLI-NIT®Electronic grade aerosol cleaner 
TF-787 does it perfectly, precisely. 

It dissolves all types of organic flux and 
safely removes grit, dirt and oxides from 
any spot and restores ohmic contact. 

Specially formulated for high 
technology applications TF-787 is 
non-inflammable,non-conductive and 
safe for computers, telecommunications 
apparatus, magnetic tape heads, 
electronic tuners and switches and low 
energy sensitive contacts. 

KLI-NIT® TF-787- a trusted companion 
for electronic professionals 


es LIMITH^ 

Sussex Road, Bombay-400 027. 
Phone:872-2888 Gram: "HAKOTRONIX' 1 


elektor india march 1985 3.05 




FIRE RETARDANT 
3^. PRINTED CIRCUIT BOARDS 

For 




Colour &B/W T.Vs. 

As Recommended by 


U Department of Electronics 

. ’ i a. i ft a .x : _ i 


1 

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Imported Material 
NEMA- FR-2/FR-3/FR-4 
GRAFICA-The name you can rely 
on for quality PCB's. 

H grafica 

DISPLAY CO., 

86, Mathuradas Vasanji Road. 

Near Darpan Cinema. Andheri (East). 
Bombay 400 093 
Phone: 6044464-6321383 

<r<r 



• Works effectively 
on single/double - 
sided PC Boards. 

• Powerful vacuum 
suction. 

• Low recoil action. 

• Dependable 
performance. 


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LOW COST 


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Engineering end Production 


queWy levels Rrlsyt from IN* 
most • asci mg to the simplest m 
operetmg vpec-hcataons are 

<*edit v ev Salable st li|knM 


isfKmsn engineers become pert 
of your design group m selecting 
the correct type of relay to meet 
ell requirements. 


Eagleman 


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3/1 9-A, Ksrti Nagar. 
Industrial Area. 

New Delhi- 11 001 5 
Phone 535770. 
Grams EAGLENT 




WE OFFER FROM STOCK 


I.C.'s : TTL. CMOS, MOS, LSI, 
Microproccessor. Micro computer etc. 

Zener Diodes : 400 mw & 1 watt 

1-^D S Red, Green, Yellow in 
5mm and 3mm dia 

1C Sockets I SMK & Memorex make 

Trimpots : 

Multiturn Bourn's, VRN & Beckman make 

Single Turn cermets : 

EC as well as imported 

Floppy Discs : 

8" as well as mini floppy of memorex, 

& dyson make 

Write to : 



Pushpdant Niwas 3rd Floor, 3. Chunam Lane. Or. D. Bhadkamkar 
Marg Bombay-400 007. Phone : 5137225. 5135B45 


3.06 elektor India march 1985 









elektor india march 1985 3.1 1 


«eV>q'^o 








Interstate U.S.A. . through exhaustive 
research and development, have 
mastered the art of voice command and 
made a fairy tale a reality. You can now 
enjoy increased efficiency, reduced 
handmade errors, thereby enhancing 
productivity and quality by voice 
commanding your Computers. 

Machines, Processors, Controllers. 
Instruments. Typewriters, even Cars — 
in fact any thing that needs man machine 
interaction. 

Why not try a "Alibaba," beginning with 
some of the voice recognition chips and 
systems available, like 1 B and 1 OO word 
command recognition chips, Total voice 
recognition systems and Speech 
recognizers compatible with DEC-VT. 
C.ITOH and Plessy PT-1 OO terminals. 
E.E.E. can offer you more details on 
Interstate products and those 
manufactured by other principals 
overseas, such as: 


Tandberg Data, Norway for CRT 
terminals and Streaming (QIC) cartridge 
tape drives 

Thandar Electronics, UK for Logic 
Analyzers with personality modules 
Trio-Kenwood, Japan for Audio-Video £» 
Entertainment electronic Test and 
Measuring equipment 
Comark Ltd., UK for Thermocouples of 
all types and grades 

You say Open Sesame' and we'll help you 
reach your riches. For further details 
write to: 


The Eastern Electric & 
Engineering Company 
Private Limited 

Gyan Ghar, Plot No. 434A, 

1 4th Road, Khar, 

Bombay 400 052. 


SELLADS/EE E/6/84 





electronics the easiest way 



Some of the most delightful obser- 
vations about electronic communi- 
cations have been boldy put to paper by 
primary school miniprofessors. Take 
these historical explanations for 
example. 

Question: "When was the radio inven- 
ted?' Answer: 'On page 24.' 

The radio was invented in the pre-me 
times.' 

The Romans did not have radios. 
They used smoke signals in both the 
A.C. and D.C. times.' 

Children have a knack for discarding 
everything but what they consider to be 
the most essential information. One boy 
brusquely wrapped up all of man's yearn- 
ings, struggles and triumphs in this eight 
word package: 'Progress was from 
electricity to radios to now.' 

Here's a remark as charming as child- 
hood itself: 'I was thinking the radio 


electronics 
die easiest way 


It is well known that children can 
generate some of the most original 
ideas on any subject. We can only 
envy the straightforward simplicity 
of their answers to questions of a 
technical nature, especially when 
the subject is electronics . . . 



3.12 eleklor India march 1985 


was invented before the telegraph. When 
I learned different, all the thoughts I 
was going to say went in a swallow 
down my throat.' 

Another tiny historian concluded: 
The Dark Ages lasted until the invention 
of electricity.' 

Through the years, the youngest 
generations' fund of knowledge has 
proved to be a glittering gold mine of 
wit and unconscious wisdom, often 
conveniently unhampered by hard facts. 
Each new subject seems to be a fertile 
new field for off-centred interpretation 
and lopsided logic. Digging into facts 
about Marconi produced such notable 
nuggets as these: 

'Marconi was born in 1874,supposably 
on his birthday.' 

'It took much hard work for Marconi 
to think out how to invent the radio. He 
had to keep thinking around the clock, 
twelve days a week.' 

'In just a few short years he became a 
sensation overnight.' 

'He expired in 1937 and later died 
from this.' 

Recently a bright-eyed little radio 
enthusiast came up with this endorse- 
ment: 'Every time I think how the radio 
gives us so much fun, I have joy feels all 
over.' 

A skeptical classmate of hers absorbed 
all the statistics regarding the number of 
ham radio operators, but got his 
skepticism across in one crushing state- 
ment: The total amount of ham oper- 
ators today is more for saying than 
believing.' 

It must run in the familiy. Two years 
later his younger sister reported: The 


number of ham operators we have today 
is an adsurbly large fact of a number.' 

The subject of hams has stumped 
many eager young scholars. Here are 
three more futile but imaginative explan- 
ations: 

'Ham operators look something like 
people.' 

'They are one of the chief by-products 
of electricity.' 

'The meaning of them has a very short 
memory in my mind.' 

The elementary school youngster's 
mind seems to be a vast storehouse of 
miscellaneous misinformation — half 
true, half false and wholly delightful. 
His fund of knowledge about electricity 
includes such fascinating items as these: 

'Electricity has been with us forever 
and maybe even longer.' 

'Would the average person be able to 
keep up with the news if it was not for 
electricity? The chances are 999 out of 
a hundred.' 

'In electricity, opposites attract and 
vice versa.' 

'If you see lightning, no you don't. 
You see electricity.' 

'From now on, I will put both gladness 
and wonder in my same thought about 
electricity.' 

Here's one I've been trying to figure 
out for five years: 'You should always 
capitalize the word electricity unless it 
is not the first word in the sentence.' 

This next little girl seemed to be giving 
it all she had when she wrote: 'Correct 
my being wrung, but tell me true or 
false. Do negative charges go through 
electrons or through protons? I wrecked 
my brain trying to think which. ' 

But I'm afraid others are more non- 
chalant in their pursuit of knowledge: 
'Protons are bigger than electrons in 
case I ever want to know.' 

Psychologists tell us that half learning 
a fact incorrectly is often the first step 
to learning it right. So let's be philo- 
sophical as we buzz through these 
fractured facts about electrons and 
protons: 

'100 electrons equal 1 radio program.' 

'When the switch is on, electrons are 
constantly bumping into each other 
inside the wire. There is really quite an 
overpopulation of electrons.' 

'Once I saw in an educational cartoon 
about how electrons move. Electrons 
are very interesting folks. All their ways 
are hurry ways.' 

'Electrons carry the negative charge 
while protons take the affirmative.' 

'Electrons are the same as protons 
only just the opposite.' 

'I think I admire the electron more 
than anything else about electricity 
because it weighs only about one over 
2000th as much as a proton but can still 
hold its own.' 

When questioned, children offer the 
ever present possibility that however far 
from right their answers may be, the 
next wrong answer could be more witty 
and thought-provoking than the correct 
one. Sometimes they don't know and 


electronics the easiest way 


they know they don't know, but that 
doesn't keep their answers from being 
charming: 

'Ideas about how radios work have 
advanced to the point where they are no 
longer understandable.' 

'Did I pass the test about how to get a 
ham radio operator's license and why 
not?' 

'I have found radios to be easier to 
listen to than to tell how they work.' 

Take three small boys, mix them up 
thoroughly with several pounds of 
strange facts, then shake up with an 
examination and you have the perfect 
formula for instant confusion. 

'The way vacuum tubes work, as I 
understand it, is not very well under- 
stood.' 

‘Many questions have been aroused in 
my mind about vacuum tubes. As a 
mattery fact, the main trouble with 
vacuum tubes is that they give more 
questions than answers.' 

'In electricity, positives are attracted 
by negatives for the reason of search 
me.' 

Often a grownup can only envy the 
simplicity of a child's way of expression, 
as is the case of the lass who remarked: 
'When I learned we were going to see a 
movie about ham operators all over the 
world, I told my feet to quiet down but 
they felt too Saturday to listen.' 

In their world of uncertainty, once 
they know a fact for certain, they hang 
on to it tenaciously, e.g.: 'Another name 
for the radio is radiotelephony, but I 
think I will just stick with the first name 
and learn it good.' 

Children, like mountain climbers, must 
always make sure that their grasp on a 
fact is firm, even though they want to 
leap far beyond. Otherwise, they may 
find themselves trapped on a mental 
ledge. There is usually at least an 
element of truth in the most absurd 
answer. Sometimes they aren't wrong at 
all. It's just the way they put it that's so 
funny: 

'Radio has a plural known as mass 
communication.' 

'Water scientists have figured out how 
to change river currents into electric 
currents.' 

'The best thing live wires are good for 
is running away from.' 

'Quite a bit of the world's supply of 
electricity goes into the making of ham 
radios.' 

'Many things about electronic com- 
munication that were once thought to 
be science fiction now actually are.' 

Members of the primary school set 
certainly have their own opinions, and 
few are hesitant to express them: 

'All the stuff inside a ham radio is so 
twisted and complicated it is really not 
good for anything but being the stuff 
inside a ham radio.' 

'Electronics is the study of how to get 
electricity without lightning.' 

How about this unforgetable remark: 
'Last month I found out how a radio 
works by taking it apart. I both found 


out and got in trouble'. 

And you can't argue with the young 
fellow who reported: 'When currents at 
200 to 240 volts go through them 
radios start making sounds. So would 
anybody.' 

Just what is a vacuum? Here are five 
answers, fresh from the minds of nine- 
year-olds: 

'Vacuums are made up mostly of 
nothings.' 

'A vacuum is an empty place with 
nothing in it.’ 

'Vacuums are not anythings. We only 
mention them to let them know we 
know they're there.' 

'There is no air in vacuums. That 
means there is nothing. Try to think of 
it. It is easier to think of anything than 
nothing.' 

‘A vacuum tube contains nothing. All 
of its parts are outside of itself.' 

Another lad wrote of this frustrating 
experience: 'I figured out how a vacuum 
tube works twice but I forgot it three 
times.' 

One of his classmates reported: ‘When 
I learned how empty vacuum tubes are, 

I would have fainted if I knew how.' 

If you're at all hazy about other parts 
in a radio, hang on. These next thoughts 
will leave you only slightly worse off 
than before: 

'An electron tube can be heated two 
different ways. Either Fahrenheit or 
Centipede.' 

'When you turn a radio on, the tubes 
get hot. The hotter anything gets, the 
faster the molecules in it move. Like if a 
person sits on something hot, his 
molecules tell him to get up quick.' 

‘In finding out that radio tubes get 
hot, the fun is not in the fingers.' 

Transistors are what cause many 
radios to play. Transistors are a small 
but important occupation.' 

'We now have radios that can run on 
either standard or daylight time.' 

One student had many tussles with 
his spelling book. When he finished 
writing one particular sentence, the 
battleground looked like this: 'ter- 
manuls do not agree with themselves 
spelingly and pruncingly.' 

With apologies to Mr. Webster, I would 
like to present a pocket-size dictionary 
of pint-size definitions, compiled from 
school children's reports. Should any 
of them prompt Webster to turn over in 
his grave, he would have to do so with a 
smile: 

'Axually, a choke coil is not as danger- 
ous as its name sounds.' 

'Electromagnets are what you get from 
mixing electricity and magnets together.' 

'Think of a volt. Then yippee, because 
now you have had the same thought as 
Voltaire, after who this thought was 
named.' 

Another lad had the right information, 
but the wrong answer: There are some 
things about electricity we are still not 
sure of. These things are called whats.' 

If the kids don't know all the answers, 
they can always do what their parents 


once did — try to slide by on a guess or 
two: 

'A radio telescope is a thing you can 
hear programs by looking through it.' 

'Current electricity is electricity that is 
currently in use.' 

Children are so full of questions, they 
can't possibly wait for someone to tell 
them all the answers. That's why they 
plunge recklessly ahead on their own, 
like so: 

'Sound travels better in water than in 
air because in water the molecules are 
much closer apart.' 

'I have noticed that if a portable radio 
is turned in different directions, the 
station talks loudest behind its back.' 

'Although air is hollow it is not just 
for looking through. It is also for having 
radio waves running through it and 
trying to answer questions about.' 

'Radio waves would not be all that 
important to study if it were not for 
ears.' 

'Someone in here said that FM has 
shorter waves than shortwave radios. Is 
this so? I think it is because I think I 
was the one that said it.' (If you can't 
believe yourself these days, who can 
you believe?) 

An obviously more confident young 
man proclaimed' 'Much has been said 
about how radio waves travel. Radio 
waves are both hearable and talkable.' 

The last word must go to this moppet 
who was doing well — until the last word: 

'I believe the radio is one of the most 
important inventions of all time. Of 
course my father works at a radio 
station, so I may be a little pregnant.' 

That's one young writer who would 
have done fine if she had just stopped 
while she was ahead (which is good 
advice for grownup writers, too). 

By kind permission of 73's magazine, k 


COMING SOON X 


elektor will soon unfold the 
mysteries, that surround 
electronics, to these 
inquisitive minds. 

Our new section SELEX 
(Simple Electronics 
Experiments) Is aimed at 
students and beginners 
SELEX will teach 
"electronics the easiest way" 

Turn over (or more 
inlormation on 

s. : > 

elektor mdia march 1985 3.1 3 



Learn electronics 
the easiest way! 


What is SELEX? 


SELEX is a new section to be introduced 
shortly in elektor india, as a regular 
feature. SELEX stands for Simple Electronic 
Experiments. SELEX will teach various topics 
in electronics in a very simple and 
elementary manner! Even a layman can learn 
basics and a lot more in electronics through 
SELEX. SELEX will be the most Reader Friendly' 
section. Students and beginners who always 
had a feeling of being deprived, can now a 
shake off that feeling and get ready 
to catch up with today s incredibly 
rapid advances in electronics. 


Getting all the data about the components 
you are using is as important to the 
process of learning as the experiment 
itself. SELEX will provide all component 
data that you would need to have. 
If you are fascinated by bits and bytes, 
TTL, CMOS, ECL, NMOS, now SELEX will 
tell you all about it! A specially 
devised step by step 'Digital 
Course' will soon be introduced 
in SELEX. 


^ Watch out for \ 

SELEX 

in the conning issues 
. of elektor india. / 


You have always wondered about 

how things work? Now, SELEX 

will tell you! From simple 

test instruments to complex 

Radar systems, SELEX will 

tell you how they work. It 

will be a journey right into i; JH 

these equipments. SMB 

SELEX will tell you all about J||9sj 

raw materials and construction 

of various components, tools 

etc. like soldering irons. 

transformers, battery cells. 

active and passive components ; 

and many more things. 

If you have avoided experimenting 
with electronics for the fear of 
blowing up costly components or 
equipments, now SELEX will tell you , 

how to use components, tools and 
instruments with care and avoid 
misuse and damage. 

A student, a beginner, a hobbyist can ^ 
always reach the professional level 
with 'Hands-on' experience through SELEX. 
SELEX will bring you interestingly devised 
experiments to teach you the basic principles 
of electronics. 



the government, whose ostensible 
aim was to make available the 
components in plenty at 
internationally competitive prices 
Mr Venkatraman opined that 
there was no definite policy on 
components at all 
The prices of some Indian 
components were even now 
comparable to international prices 
but in the case of some other 
indigenous components, there 
was no possibility of achieving 
any price parity in the foreseeable 
future, he added 
The danger, according to Mr 
Venkatraman lay in the import- 
based growth as in the absence of 
real domestic base of components 
the industry would collapse. 

Bush Computers 
Bush India Limited has tied up an 
agreement with a leading US 
electronics company, General 
Automation, for the manufacture 
of micro computers. 

General Automation's director of 
sales for South East Asia. 

Mr. Carlton J Parker is quoted as 


2000 kilowatts under the 
sea 

In 1927, The Netherlands became 
one of the first countries to 
recognise the power of the short- 
wave broadcasting medium. Early 
experiments via station PCJ in Eind- 
hoven were convincing enough to 
make a solid investment in the 
future. But the shortwave dial has 
certainly changed these last 58 years 
and, to maintain and improve the 
flow of information from broadcaster 
to listener, technology has had to 
adapt too. These days, it's quite 
common to read in broadcasting or 
shortwave-listener magazines that a 
new transmitter is going on the air. 
Radio Nederland Wereldomroep's 
solution, though, has some rather 
unusual aspects to it. 

Two million amongst fourteen million 
You can’t put a shortwave transmit- 
j ter site anywhere! Not only are the 
i aerial masts up to 120 metres high, 
l but they need to radiate concen- 
| trated beams of energy into the air. 
Finding a nice secluded spot in The 
Netherlands, a small country with 
14 million people, is a difficult task. 

| In 1937, the Dutch made broad- 
casting history when they con- 
structed a wooden rotatable 
J directional shortwave aerial. It was at 
[ a place called Huizen (pronounced 
I How-zen), a few miles north-east 



saying that the collaboration 
envisaged introduction of PICK 
computer operating systems for 
the first time in India, which is 
said to be more flexible and 
interactive than the other 
prevailing systems Five models of 
the Zebra series and two others of 
the Unix operating systems are t» 
be made available to India and the 
retail price of each system is 
expected to be around Rs. 3.50 
lakhs. 

Mr. Ashok Aggarwal, marketing 
manager of Bush, has stated that 
the machines would be targetted 
mainly to the medium and large 
scale public and private sector 
units which require many 
terminals to be connected to one 
main computer for information 
sharing The company would 
initially make about 100 Zebra 
systems per annum The company 
will also manufacture home 
computers called PH-1. IBM- 
compatible personal computers 
called the Bush Attache and the 
PICK-based, IBM-compatible, 
personal computer called Bush PC. 





from the studios in Hilversum. This largest of these, Flevoland, was 
huge construction would swing pumped dry in two stages between 

round to point the aerial in different 1950 and 1968. Today it's already an 

directions. Today, an inscription in an established area for arable crops, and 
appartment block, the 'PHOHI flats' now also for shortwave broadcasting, 
marks the spot where the aerial once Radio Nederland Wereldomroep's 
stood. new 'Flevo’ transmitting centre is 

In the 1950's, shortwave broad- also an ambitious project in its own 

casting from The Netherlands moved right. To be efficient, a shortwave 
to the centre of the country, to the transmitter needs efficient directional 
village of Lopik in the province of aerial, which means that for the 
Utrecht. There was room for future lower shortwave broadcasting ba 

expansion in those days, but not such as 49 metres, this entails v. 

now. As the Lopik facilities began to large constructions. Since Flevo 

show their age, the search started four metres below sea level, t(. 

for a new place to put the shortwave water table is quite high and t 

transmitters. In fact, the solution was ground is also rather soft. New 
to start construction within a few techniques have had to be found to 
miles, as the crow flies, of the old anchor the aerial masts securely, 
Huizen aerial site. Four 500 kilowatt since the totally flat polder means 
transmitters were ordered, plus one everything is exposed to thr full 
100 kW reserve transmitter. But not force of wintry weathe- 
only is the transmitting centre new, q p j n ,h e air 
so is the land it's built on. Flevo is equipped with some so- 

The 28th of May 1932 saw the birth called 'omni-directional' aerials used 

of a new lake in The Netherlands, to serve nearby target areas in 

with a size of 1200 square kilometres. Europe. These radiate energy in all 

Completion of the so-called 'Afsluit- directions. But the days of being 
dijk', a dike some 30 kilometres long, able to serve listeners all over the 
meant part of the former Zuydersee world with one frequency are over, 
was no longer open to the wild Now, 'directional' aerials are far more 

North Sea. It was given the name important, especially to serve distant 
'Ijsselmeer’. Plans didn't stop there, target areas. So these aerials con- 
for then began an ambitious draining centrate the energy into a relatively 
scheme to create new areas of land narrow beam. This not only gives a 
previously covered by the sea. The stronger signal in the chosen target 


eleklor India march 1 985 3.1 7 





area, but it means that interference 
to other stations, serving different 
parts of the world on the same fre- 
quency, is reduced to a minimum. 
This in turn contributes to less over- 
crowding of the shortwave spectrum. 
Aerial design is a specialized part of 
engineering technology. A directional 
aerial is more than a simple dipole 
strung between two supporting 
towers. In fact, most of the Flevo 
aerials consist of sixteen dipoles, 
arranged in four rows, each of four 
dipoles, forming a so-called 'curtain 
array'. A screen of horizontally strung 
metal wires is put behind the stack 
of dipoles, acting rather like a mirror. 
This ensures that energy is radiated 
in one direction only. The size of the 
dipoles is important, as some aerials 
are designed only to operate on four 
out of the total of nine shortwave 
bands used by Flevo for international 
broadcasting. If you try to operate 
an aerial on frequencies outside the 
ones it's designed for, it will not 
match electrically. Energy is then 
reflected back into the transmitter, 
and generally lost as excess heat. 
Since Flevo uses about 3.5 million 
watts from the mains electricity 
(think of it as paying the electricity 
bill for 35 000 light bulbs), it's 
important that as much of this 
energy as possible is used for broad- 
casting programmes. 

Whilst computer programs exist to 
calculate how a chosen aerial design 
SHOULD perform in theory, a lot of 
natural or man-made factors (like the 
type of soil, nearby metal aerial 
towers, etc.) also have to be con- 
sidered in practice. So, having hung 
the aerials between the supporting 
towers, the Dutch PTT hired a 
helicopter equipped with special 
measuring apparatus, and switched 
the transmitter on with reduced 
power (20 kW). By flying in a circle 
with a radius of 2 kilometres from 
the aerials, it was then possible to 
plot the radiation patterns of each 
aerial. At a height of 500 metres, the 
beam direction is measured to within 
2 degrees, together with the beam 
width and elevation. 

The exact direction an aerial will 
beam to depends mainly on its 
physical orientation on the ground. 
The 'star' shape of the Flevo aerial 
complex means that all directions of 
the compass between 050 and 
290 degrees can be reached. It’s also 
possible to electrically change the 
beam direction of some aerials. If an 
aerial normally beams due east 
(equivalent to 090 degrees) it can be 
adjusted to operate at 060, 075, 105 
and 120 degrees as well. Changing 
the direction more than this would 
lead to undesirable energy loss in 


unwanted directions. 

No aerial can be one hundred per 
cent efficient. As well as beaming 
energy in the desired direction, some 
signal will also go in the opposite 
direction. This is termed 'back- 
radiation'. If, for example, 

500 kilowatts is beamed one way, as 
much as 50 kilowatts is often sent 
the other way. By design and careful 
measurements at Flevo, this back 
radiation has been reduced to a 
minimum. The ratio of radiated 
energy at the front of the antenna, 
against the power measured at the 
back, is now as high as 20 dB. This 
means that only around 5 kilowatts 
are radiated into the opposite 
unwanted direction. 

All these factors are important in 
ensuring that the energy isn’t 
wasted. Flevo is believed to be the 
first shortwave station where such 
intense aerial diagram measurements 
have been done from the air, before 
the transmitter site enters service. 
With such high powers being used, 
the feeder lines to the aerial have 
had to be covered. At previous 
transmitting sites these were simply 
bare wires on poles, but since they 
offer a potentially lethal hazard to 
birds, extra precautions were taken 
with the new project. These feeders 
are now constructed of coaxial cable, 
which means that high voltage areas 
are screened. 

On the ground 

The transmitter design also contains 
some new concepts. Since short- 
wave broadcasting began, a system 
known as Amplitude Modulation, 

AM, has been used to get the signal 
from transmitter to receiver. The AM 
signal involves two components: 

1. The 'carrier' which puts the signal 
on a certain part of the shortwave 

dial, and is needed by the shortwave 
receiver as a sort of 'reference point'. 

2. The modulation, which is actually 


the speech and music information 
the broadcaster wants to put across. 
The problem is that more than fifty 
per cent of transmitter energy is put 
into the carrier part of the signal, 
which in fact contains no programme 
information at all. Ways around this 
are planned for the future, with more 
efficient forms of transmitting tech- 
niques, but most require that the 
listener buys a new type of radio. 

This isn't practical yet. But modern 
transmitter design enables the use of 
a more efficient form of AM, known 
as Dynamic Amplitude Modulation 
(DAM). With normal AM in 
widespread use today, the level of 
the carrier remains at a constant 
level. In the DAM technique, the car 
rier power moves in step with the 
modulation. So, during a loud piece 
of music the carrier power is turned 
up, but when the music gets softer, 
the carrier power is turned down. 

This is done electronically, and can 
mean anything up to a twenty-five 
per cent energy saving or more! This 
is achieved without a noticable qual- 
ity reduction of the signal at the 
listener's end. The use of DAM can 
be noted on the signal strength 
meter of a shortwave radio, the 
needle moving in step with the pro- 
gramme being listened to. 

This DAM technique, together with 
other energy saving designs incor- 
porated into the transmitters, means 
that while the total power output of 
Flevo is 5 times that of Lopik, the 
power bill is expected to rise by only 
about 2.5 times for the same hours 
of usage. The transmitters are cooled 
both by water and air systems. Three 
hundred litres of water per minute 
passes through each sender, and the 
excess hot air is used to heat the 
building. 

Computer technology is also used to 
the maximum. Changing frequencies 
at the old Lopik transmitter facilities 
was quite an ordeal. Moving from 


3.18 elektor India march 1 985 







one band to another often entailed 
physically moving and tuning quite a 
number of parts of the transmitter. 

It's a credit to the transmitter crews 
that they managed to do this with 
the required precision in the short 
time available between programmes. 
Modern multi band transmitters have 
eliminated the need for this type of 
manual labour. But engineering skill 
is now focussed instead on maintain- 
ing a highly complex computer con- 
trolled switching system. New pro- 
gramme and frequency schedules are 
entered into a computer terminal at 
Radio Nederland Wereldomroep, 
where it's possible to monitor what’s 
happening some 16 kilometres away. 

The start of a new era 
The testing phase of the transmitter 
complex is now nearing completion. 
A new programme and frequency 
schedule will commence on 31 March 
1985 taking advantage of the ability 
to serve new areas of the world with 
a stronger signal. 

The philosophy of Radio Nederland 
remains unaltered. As a non- 
commercial public foundation, 
financed from the Dutch radio-tv 
license fee, its aim is to bridge the 
information gap between this part of 
Europe and the rest of the world. 
This is done by not only examining 
one's own point of view, but also 
those in the listener's region. Only 
then can one speak of 'communi- 
cation'. 

From dream to reality! 

If you want to tune in the world, 
you really only need three things: a 
pair of ears, a shortwave radio, and 
an aerial. Getting a good shortwave 
receiver is less of a problem these 
days, but most shortwave listeners 
and radio amateurs wish they had 
more space to put up a better aerial. 
After all, you can own the world's 
best receiver, but without a suitable 
aerial, all you'll hear is interference 
and a few of the stronger signals. 
Radio Nederland Wereldomroep (or 
Radio Netherlands as it is called in 
English) is a shortwave broadcasting 
station, based in Hilversum, The 
Netherlands. It has built up a unique 
consumer information database on 
shortwave receiving equipment, 
publications, and accessories, with 
the aim of assisting shortwave 
listeners around the world. As the 
preceding section explains, the 
station will shortly have a new trans- 
mitting centre located on the Flevo- 
polder. But, before it enters service 
on 31 March 1985, the Flevo site will 
be the location of a unique amateur 
radio experiment. 



On the third weekend in February, 
two ordinary amateur radio transmit- 
ters will be taken out to the new 
transmitter site. The transmitters will 
be set up as usual, following the 
requirements laid down by the Dutch 
PTT licensing authorities. The differ- 
ence is that these transmitters will be 
connected to some of the largest 
directional shortwave aerials in the 
world! The plan is to use the new 
Flevo transmitting site aerials ON 
AMATEUR RADIO FREQUENCIES 
for a period of 36 hours. Not only 
will this be a unique chance for the 
operators to work with such high- 
gain aerials and examine the results, 
but it offers a rare opportunity for 
radio amateurs and shortwave 
listeners to listen out for a station 
with a difference! This is about as 
close as possible to the shortwave 
enthusiasts dream station equipment. 
The amateur radio station will be on 
the air between 0600 GMT on Satur- 
day 16 February 1985 and 1800 GMT 
on Sunday 17 February 1985. This is 
a continuous period of 36 hours of 
operation. One transmitter will 
operate on a non-directional aerial, 
intended for European reception. The 
second will make full use of the 
giant curtain arrays at the Flevo 
shortwave transmitter site. The direc- 
tion of the beam will follow the pat- 
tern of the regular English language 
broadcasts from Radio Netherlands, 
i.e. at 0730 GMT, when Radio 
Netherlands is on the air to Australia 
and New Zealand, the amateur radio 
station will beam in that direction 
too, though on a different part of the 
shortwave dial. 

The special event amateur radio 
station will operate in single side- 
band (SSB) and CW (Morse) modes. 
The Dutch PTT has allocated the 
special call sign 'PA6FLD' for this 
occasion. A special QSL card, depic- 
ting the new Flevo transmitter site, 
and the amateur radio operation, will 


be sent to all those submitting cor- 
rect reception reports. Licensed radio 
amateurs will, of course, be able to 
talk directly to the operators at the 
station. But shortwave listeners are 
encouraged to look for the station 
too. Exact frequencies for the 
amateur radio stations will be 
announced nearer the date, during 
the "Media Network" programme. 
Details of this are listed below. 

Special Radio Netherlands English 
language programmes too! 

Between 0730 GMT on Saturday 16 
February 1985 and 0630 GMT on 
Sunday 17 February 1985, Radio 
Netherlands' regular English language 
programmes will pay special atten- 
tion to this amateur radio event. 
Several transmissions will originate 
live from the amateur radio shack at 
Flevo to watch the progress. Atten- 
tion will also be given to the 
development of the Flevo transmitter 
site and the polder in which it is 
built. Interviews with members of the 
Dutch amateur radio community and 
the PTT are also envisaged. This 
special programme can be heard at 
the times and frequencies given 
below. 

Further details of this event will be 
announced in the regular weekly 
shortwave communications magazine 
programme "Media Network". This is 
heard each Thursday on Radio 
Netherlands' English Service, at the 
same times and frequencies listed 
below. 

For any further information please 
contact: 

Jonathan Marks 
English Section, 

Radio Nederland Wereldomroep 
P.O. Box 222, 

1200 JG Hilversum 
The Netherlands 

Tel: (31) 35 16151 (ext 344) (Mon-Fri 
0800-1600 GMT) (959 S) 


Saturday 16 February 1985 


Time (GMT) 

Frequency (kHz) 


Beamed to 

0730 

9770, 9715 


Australasia 

0930 

15560, 11930, 9895, 

6045, 5955 

Europe 

1030 

9650, 6020 


Australasia + Caribbean 

1330 

17605, 11935, 9895, 

6020, 5955 

Europe 

1430 

21480, 17605, 11735 


South-East Asia 

1830 

9540, 6020 


East/Southern Africa + 
Europe 

2030 

17605, 15560, 11740, 

11730, 9540 

West Africa (also audible in 
Europe) 


Sunday 17 February 1985 (still Saturday night in the listener's area). 


0230 9590, 6165 East Coast North America 

0530 9715, 6165 West Coast North America 


elector India march 1985 3.19 


gyroflash 



from an idea 
by F. Lemoine 


gyroflash 


Every once in a while some Elektor reader sends us a circuit that 
does not fall into any of our standard categories but is none the less 
interesting. This gyroflash is just such a design. It consists of five 
xenon tubes that flash one after another so that the light moves 
around in a circle as in a lighthouse. Call it what you will — party 
trick, novelty or simply 'flashing thingummyjig' — the gyroflash is an 
interesting design idea. The fact that it can also be of practical use 
simply makes it all the more interesting. 


five sequential 
rotary flash 
tubes controlled 
by a single 
circuit 


Practical projects have always occupied 
most of the space in any issue of Elektor. 
Every now and again, however, an 
interesting design appears and is found to 
be not very practical from the point of 
view of how useful it is. Gyroflash is an 
interesting design that also has a practical 
value. It could be considered as a slave 
flash and with a very long exposure time 
this could probably provide some very 
interesting results. Avid scale modellers 
could use it as the base for a sophisti- 
cated miniature lighthouse. Others may 
like to use it as a simple novelty or build a 
party trick around it. It could also be used 
as a traffic warning or distress light as it 
does not need a mains power supply. 

What appealed to us, however, is not the 
use but the circuit itself, which is 
interesting purely as a design idea. 

The circuit 

The most obvious characteristic of the cir- 
cuit, which is shown in figure 1, is the 


repetition: the same circuitry is used five 
times to drive the five flash tubes. A pair 
of transistors, T1 and T2, and transformer 
Tr6 form an oscillator generating a fre- 
quency of about 50 to 60 Hz. The tran- 
sistors are protected by diodes D1 and D2 
(— U BE) as well as D3 and D4 (UCEmax)- 
The voltage at the secondary winding of 
Tr6 is rectified by D5 . . . D8 and Cl to 
about 250 to 300 V d.c. Variations in load 
are smoothed by capacitor Cl. The actual 
charge that ignites the xenon flash tubes 
is stored by electrolytic C2. A resistor, Rl, 
is included between these two elec- 
trolytics to prevent Cl from being affected 
by the discharging of C2. If a higher out- 
put is needed for the xenon tubes Rl can 
be replaced by a suitable coil, such as the 
primary winding (a.c. side) of a 10 VA 
transformer. If the circuit is only used for 
short periods of time the resistor is the 
better choice. The inductor should be 
used if the gyroflash must operate con- 
tinuously for a long time. 


3.20 elektor mdia march 1985 



The driver stages 

The five driver stages are, as we have 
already said, identical so we will simply 
consider one as an example. In the 
quiescent state capacitor C3 charges up 
to about 100 V via R4 and one of the wind- 
ings of Trl. When a logic T (+12 V) is 
applied to the base of T3 this transistor 
conducts and triggers thyristor Thl. The 
220 n capacitor, C3, then discharges 
through Thl and Trl. The high voltage that 
appears across the secondary winding of 
the transformer triggers Lai. The xenon 
gas inside the tube is ionised and is 
therefore conductive, with the result that 
capacitor C2 discharges through Lai and 
causes it to flash. 

A pulse generator 

We have just seen how the driver stages 
cause the xenon tubes to flash but have 
skimmed over one important point, 
namely how the stages are themselves 
triggered. A second oscillator, based on 
IC2, is used for this. Its frequency can be 
preset with PI to between 1 and 4 Hz. The 
output of the oscillator (pin 3) clocks IC1 
and this 4017 enables each of outputs 
Q0 . . ,Q4 in turn. When 05 is enabled IC1 
is immediately reset. As each of the out- 
puts goes high it triggers the transistor in 
the driver stage and the tube (one of 
Lai . . . La5) flashes. In this way the xenon 


tubes flash in tum at a speed determined 
by the setting of preset PI. The two ICs in 
this pulse generator stage are protected 
against excessively high voltages and 
noise by R21, C9 and D14. 

The gyroflash requires an external power 
supply providing a stabilised + 12 V d.c. 

A 12 V car battery is ideal but mains oper- 
ation is also possible. In this case Tr6 and 
all components to the left of it are 
replaced by a suitable isolation trans- 
former (winding ratio 1:1, 220 V/50 VA) con- 
nected straight to the mains. Current 
consumption depends on the frequency at 
which the circuit operates. If IC2 is 
oscillating at 1 Hz about 1.2 A is needed 
but if the frequency is increased to 10 Hz 
(in which case C8 will have to be reduced 
to 4.7 pF) the current consumption rises to 
2.5 A. 


Construction 

The gyroflash, as shown in the photograph, 
is assembled on four printed circuit 
boards, three of which are very simple. 
The three circular boards serve to inter- 
connect xenon tubes and trigger 
transformers and to hold them in place. 
Wires from the driver stages are con- 
nected to the lowest board. The high volt- 
age wire (+ +) feeds through both lower 
boards and connects to the anodes of 
Lai . . . La5 on the upper board. In the 


Figure 1. The most 
obvious part of this cir- 
cuit is the five identical 
flash-tube driver stages. 
The rest of the circuit 
provides a high voltage 
for these stages or trig- 
gers them at the right 
time. Note that tran- 
sistors T1 and T2 do not 
need to be fitted with 
heatsinks unless they are 
cased into a very con- 
fined space. 


eleklor indta march 1985 3.21 



gyroflash 



Figure 2. The principal 
printed circuit board of 
the gyroflash. Again it is 
the repetition of the 
driver stages that stands 
out. Never work on this 
board unless you are 
absolutely sure that 
capacitors Cl and C2 are 
completely discharged. 
This can be done by 
bridging each of their ter- 
minals in turn with a 
length of insulated cable. 


Parts list 


Resistors: 


Capacitors: 


R1.R2 = 100 Q/5 W 
R3 = 1 k/5 W* 
R4.R7.R10, 

R13.R16 = 100 k 
R5.R8.R11. 

R14,R17 = 470 Q 
R6.R9TR12. 

R15.R18 = 270 Q 
R19.R20 - 10 k 
R21 = 100 Q 
PI = 100 k preset 


Cl = 50 m/350 V 
C2 = 16 m/350 V* 

C3. . .C7 = 220 n/400 V 
C8 = 10 p/16 V 
C9 = 47 m/16 V 


\ 


Semiconductors: 

D1,D2 = 1N4001 
D3.D4 = 47 V/1 W zener 
D5. D8 = 1N4007 
D9. . D13 = 100 V/ 

1 W zener 

D14 = 15 V/1 W zener 
T1,T2 - BD 241 C 
T3...T7 = BC547B 
Thl. . Th5 = TIC 106D 
I Cl - 4017 
IC2 = 555 


Miscellaneous: 

LI = * 

Lai . . . La5 xenon flash 
tubes 

Tr1...Tr5 = trigger 
transformers for Lai . . . La5 

Tr6 = transformer, primary 
2 x 9 V/1 A, 
secondary 240 V 

* = see text 


3.22 elekior mdia march 1985 









same way the ground line travels via the 
lower board to the middle one where it is 
linked to the cathodes of the xenon tubes. 
Neither of these wires is visible in the 
photograph as they are fed behind the 
mirrors we have used to enhance the 
appearance of the gyroflash. 

The usual rules of construction apply for 
this circuit. Work carefully and there 
should be no problem. Most of the wiring 
between the various boards carries high 
voltage and/or high current so make sure 
the cable used is thick enough to with- 
stand the load. NEVER WORK ON THE 
CIRCUIT WITHOUT FIRST DISCHARGING 
CAPACITORS Cl AND C2. Failing to do 
this can quite literally be lethal. 

When the circuit is constructed and wired 
up it must be calibrated. All this involves 
is setting preset PI so that the tubes flash 
at the frequency you find best. If the 
maximum frequency is not fast enough for 
your purposes the value of capacitor C8 
can be reduced to 4.7 fiF. 

The size of the flashing element is deter- 
mined by the length of the xenon tubes 


and the size of the trigger transformers. 
Provided the tubes are matched to the 
transformers the electrical specifications 
of both are of little consequence except 
that the transformer primary voltage must 
be between about 250 and 300 V. (A 
suitable combination of transformer and 
xenon tube is advertised in the opto- 
electrical section of the Maplin catalogue.) 
We 'decorated' the gyroflash prototype to 
improve the effect generated. This was 
quite simply done by fitting a highly- 
polished piece of thin metal behind each 
of the flash tubes of reflect its light. The 
five metal plates were soldered at the rear 
to hold them together. The gyroflash can, 
of course, be embellished or cased to suit 
the purpose to which it is put. Whatever 
the purpose and whatever type of case is 
used, one point cannot be too strongly 
stressed. Use a well-insulated case as this 
is just the sort of project that attracts pry- 
ing fingers. These prying fingers might 
not survive a shock from a wire carrying 
240 V (or more)! M 


Figure 3. These three cir- 
cular printed circuit 
boards hold the xenon 
tubes and trigger 
transformers in place and 
interconnect them elec- 
trically. If any layout 
other than this circular 
one is desired these 
boards can, of course, be 
discarded. 


elektor india march 1985 3 23 



1.2 GHz 

input stage 


The different sections 

We will start at the LF stage shown in 
figure 1. At the input is a dual-gate 
MOSFET connected as a source follower. 
A current source (T2) is included in the 
source line to minimise the attenuation 
caused by Tl. A MOSFET was used 


on the main printed circuit board of the 
frequency counter depending on whether 
the prescaler is included or not. We will 
return to this point later but let us start at 
the beginning, with the ‘lowest’ part of the 
input stage. 


One important part of the microprocessor-controlled frequency meter 
described in the February 1985 issue of Elektor India is still missing: 
the input stage. This largely decides the frequency range that can be 
measured and the sensitivity of the input. Because of its importance 
a lot of time has been spent on its design. The result is an 
instrument with a large frequency range (0.01 Hz to 1.2 GHz) and 
excellent sensitivity of 10 mVrms from 10 Hz to 100 MHz and 
100 mVrms up to 1.2 GHz. These are very respectable values and 
make the frequency counter suitable for almost every situation that 
might arise. 


super range for 

frequency 

meters 


The input stage described here was 
designed especially for our new fre- 
quency meter but it could also be modi- 
fied to suit other frequency counters. The 
layout of the input stage must be borne in 
mind, however, especially the fact that it 
has three inputs. These are: 

■ A low frequency (LF) input for analogue 
signals from 10 Hz to 10 MHz. The sensi- 
tivity can be set with a potentiometer. 

■ A digital input for CMOS and TTL 
signals up to 10 MHz. 

■ A high frequency (HF) input consisting 
of two sections, namely a HF amplifier 

for frequencies up to 100 MHz and a 
prescaler that goes from 100 MHz up to 
1.2 GHz. The ‘normal’ HF signal is divided 
by 16 and the prescaler signal by 512. 

Each user can tailor the input stage to his 
own needs. If it is used with the 
microprocessor-controlled frequency 
counter we recommend that at least the 
sections for the three inputs be built as 
they are present on the front panel and 
they are catered for in the processor sec- 
tion. If no frequencies above 100 MHz are 
to be measured the prescaler IC and 
associated divider can be omitted. One of 
two wire bridges (PR or PR) must be fitted 


because of its minimal input capacitance. 
The advantage of this is that quite a large 
resistance (R1 = 5k6) can be used for 
input protection without reducing the sen- 
sitivity of the circuit at high frequencies. 
Together with the zener diode integrated 
in the MOSFET, R1 protects the input 
against excessive voltages up to about 
100 Vpp. The impedance of the source 
follower is determined almost entirely by 
R2 and R3, which means that it is 
4M7/2 = 2M35. 

The signal travels from the source via 
capacitor C2 to IC1. This video op-amp is 
set to an amplification factor of 200 x as 
pins 4 and 11 are connected together. In 
this configuration the 733 can process fre- 
quencies up to about 40 MHz so this is 
perfectly acceptable for the 10 MHz range. 
The output signal from IC1 is fed to 
schmitt triggers N1 and N2 which form a 
clean TTL signal with steep edges and 
this is then fed to the frequency counter. 
The d.c. level at pin 1 of N1 can be set 
with P2 thereby trimming this section to 
maximum sensitivity. 

A FET, T3, connected across the inputs of 
the op-amp and with its base linked to a 
potentiometer enables ICl’s gain to be 


3.24 elektor india march 1985 



changed within certain limits. The poten- 
tiometer in question is, of course, PI, the 
sensitivity control on the frequency 
counter’s front panel. If the gate voltage is 
set to —5 V the FET is turned off and the 
circuit operates as if it was not there. As 
the magnitude of the negative gate 
voltage is reduced T3 conducts more and 
more with the result that part of the signal 
from pin 14 of IC1 is also present on pin 1. 
As with any op-amp, the 733 amplifies the 
difference between the signals at both 
inputs so the output at pin 8 decreases 
the more T3 conducts. In this way the sen- 
sitivity can be varied by a factor of 20. 
Note that T3 must be a BF246A (a BF247A 
would also work but this has a different 
pin layout). 

The input sensitivity of the stage shown 
here is at least 10 mVrms in the range 
from 10 Hz to 10 MHz. In our prototype the 
values measured were even better: 

5 mVrms between 20 Hz and 5 MHz. The 
range actually extended to 18 MHz at a 
sensitivity of 25 mVrms- 
Next we get the digital stage shown in 
figure 2. In principle the digital signals 
could also be applied to the A input but 
the signals’ large amplitudes and steep 
edges could result in an occasional incor- 
rect measurement. For this reason it is 
necessary to have a special input section 
for digital signals. The TTL or CMOS 
signals travel via an emitter follower (T4) to 
limiter circuit R14/D4/T5. The input to N3 
can never be less than —0.6 V because of 


D4, nor can it rise above 3.5 V as at this 
voltage T5 conducts and shorts pin 13 of 
N3 to ground. The edges of the signal are 
reshaped by N3 and N4 and it is then fed 
to the counter. This input is suitable for 
digital (TTL and CMOS) signals up to 15 V. 
There is an interesting point to note about 
the combination of inputs A and B. The A 
input is very sensitive and has a high 
input impedance so this stage will also 
react to signals applied to input B. This 
will be seen as a reading on the meter 
when A is chosen with the menu but the 
signal is applied to B. This is not an indi- 
cation of a fault and can cause absolutely 
no harm. The sensitivity of input A can be 
reduced to minimum by means of PI to 
get rid of the phenomenon but this is not 
essential. 

The third stage is the HF input connected 
to input C (figure 3). In this case the input 
signal is fed straight to video op-amp IC3. 
The input impedance is about 50 Q, as it 
should be for HF applications. A second 
op-amp, IC4, immediately follows the first 
and the combined amplification of the 
pair is about 50 times. The signal output 
by IC4 is divided by 16 in flip-flops 
FF1 . . . FF4 and is then passed to the 
counter. The sensitivity of this stage is at 
least 10 mVrms in the range of 10 MHz to 
100 MHz provided IC5 (FF1 and FF2) is a 
74AS74 or 74F74. If a 74S74 is used for IC5 
the sensitivity deteriorates in the region of 
100 MHz. When we used a 74F74 in our 
prototype we measured right up to 


1 




1.2 GHz input stage 


Figure 1. This is the input 
section for analogue 
input A. The signal is fed 
via source follower T1 to 
an op-amp that amplifies 
it by 200 times. 


Figure 2. The circuit for 
the digital input is very 
straightforward. An emit- 
ter follower (T1) precedes 
I a voltage limiter (con- 
sisting of R14, D4 and 
T5). The edges of the 
i signal are then cleaned 
up again by N3 and N4. 


elektor india march 1985 3.25 


1.2 GHz input stage 


Figure 3. This stage is 
used for signals above 
10 MHz (input C). Signals 
up to 100 MHz are 
amplified by IC3 and IC4 
and then divided by 16 in 
FF1 . . . FF4. From 100 MHz 
up to 1.2 GHz is handled 
by IC7. The combination 
of IC7 and IC8 divides 
these high frequency 
signals by 512. 


3 


5 V 



140 MHz at a sensitivity of 30 mVrms. In 
order to achieve maximum sensitivity at 
high frequency capacitors C26 and C27 
(shown with an asterisk in the circuit 
diagram) must be soldered directly to the 
pins of the ICs on the component side. 
This is not shown in the photograph of our 
(old) prototype but is essential because 
the capacitance is so small in both cases 
(2p2 and lp5). 

Inputs A and C combined now cover the 
frequency range up to 100 MHz. To cater 
for signals between 100 MHz and 1.2 GHz 
a special IC is needed: the SP8755 high- 
speed prescaler from Plessey. Input C is 
connected straight to this IC, which then 
divides the signal by 64. The 74LS93 (IC8) 
then divides the signal output from IC7 by 
8. In total this gives a division by 512. The 


input sensitivity of the prescaler is about 
100 mVrms- If no signals above 100 MHz 
are to be measured IC7 and IC8 can be 
omitted from the board. On the frequency 
counter’s main board wire bridge PR 
rather than PR should then be soldered in 
place. 

There are a few important points about 
the C input that should be borne in mind. 
This stage is not protected against 
excessively high voltages as this is vir- 
tually impossible at such high fre- 
quencies. The maximum input voltage 
should therefore be 5 Vpp (about 
1.7 Vrms). On the other hand the signal 
fed to the prescaler should not be too 
small. In this case the IC would give a 
stable output but the division factor might 
be 32, for example, instead of 64. The bot- 



3.26 elektor india march 1985 









1.2 GHz input stage 


tom line is always to be careful with the 
amplitude of the input signal when the 
prescaler is in use. 

Construction 

Assembling this fairly small printed circuit 
board will not be a problem providing the 
following points are followed. 

A large number of the components must 
be soldered on both sides of the board, 
wherever there is a copper island, in fact. 
It is advisable to fit these parts onto the 
board first: 


■ C4, C5 (2 x). C6, C7, C8 (2 x), C15, C16, 
C17, C18, C19 (2 x), C20, C21, C22, C23, 
C24 

■ R3, R4, R6, R7, R8, R16, R19, R20, R22, 
R25 

■ P2, P3, P4 

■ D4, T5 

■ soldering pins at + + , — 5 V, X, a and 
again 1 (at A, B and C). 

All components must be fitted as close to 
the board as possible and all interconnec- 
ting wires must be kept as short as is 
feasible. 

Shorten the soldering pins at inputs A, B 




Figure 4. The printed cir- 
cuit board for the input 
stage is double sided. The 
holes are not through 
plated, however, so a 
number of components, 
as indicated in the text, 
must be soldered at both 
sides of the board. 


Parts list 

Resistors: 

(all 1/8 W) 

R1 = 5k6 
R2, R3 - 4M7 
R4 = 3M3 
R5, R24 = 2k2 
R6 = 180 Q 

R7, R8, R12, R17 = 1 k 
R9 = 1 M 

RIO, R13. . R15, R21, 

R25 = 470 Q 
R 1 1 = 100 Q 

R16, R19, R20, R22 = 56 Q 
R18 = 15 k 
R23 =560 Q 

PI = 10 k lin. pot (16 mm 
diameter, 4 mm spindle) 

P2 = 1 k preset 
P3 = 10 k preset 
P4 = 2k5 preset 

Capacitors: 

Cl. C6, C7, CIO, 

C12. . .C19, C21, 

C25 = 10 n ceramic 
C2, C3 = 22 p/10 V Ta 
C4 330 n MKT 
C5, C20 = 10p/10 V Ta 
C8, C9, C22 - 47 n ceramic 
C11. C23, C24 = 1 n 
ceramic 
C26 = 2p2* 

C27 = 1p5* 

Semiconductors: 

D1 . . D5 = 1N4148 
T1 = BF907. BF961 
T2 = BC547B 
T3 = BF246A 
T4 - 2N2219A 
T5 = BSX20 
IC1, ICS, IC4 - 733 
IC2 - 74LS132 
IC5 = 74AS74, 74F74 
IC6 = 74LS74 
IC7 = SP8755 
IC8 = 74LS93 


Miscellaneous: 

3 off BNC chassis sockets 
(screwed fitting) 

* = see text' 


elektor indie march 1985 


3.27 


1.2 GHz input stage 


5 


Figure 5. These are the 
dimensions for the 
bracket upon which the 
printed circuit board is 
mounted. The metal must- 
be bent upward along the 
dotted line. 



bracket by means of small spacers, nuts 
and bolts. The bolts should be soldered to 
the ground line on the printed circuit 
board so that the bracket is well 
grounded. 

The whole assembly can now be fixed to 
the main board by means of the two self- 
tapping screws that help keep the main 
board in place. Make the connections 
between main and input boards (K3). The 
three BNC sockets can also be linked to 
the input board by means of three very 
short lengths of wire. The sensitivity pot is 
connected to the board with three lengths 
of wire. A heatsink is fitted to IC21 (a 
piece of aluminium of about 40 x 40 mm 
is sufficient) and the frequency meter can 
then be switched on to enable the input 
stage to be adjusted. 


6 



Figure 6. All the fre- 
quency counter's func- 
tions are given in this 
menu. A choice is made 
by pressing the buttons 
(shown above the menu) 
on the front panel. 



and C to about 2 mm (at the component 
side). The remaining components can now 
be mounted. Do not use sockets for the 
ICs; it is better to solder them directly 
onto the board. Make sure that C4 is not 
shorted to ground (at the side that is not 
connected to ground, we mean). 

The printed circuit board can now be 
mounted in the case for the frequency 
counter. To do this we must first make a 
mounting bracket from a piece of thin 
metal. The dimensions for this are shown 
in figure 5. The metal should be bent at 
90° along the dotted line. The small side 
of the bracket would stick vertically up if 
it were laid on top of figure 5. The printed 
circuit board is now mounted on the 


Purely as an aside, the current consump- 
tion of the input stage with the SP87SS is 
about 150 mA at +5 V and 70 mA at —5 V. 
Without the prescaler it becomes roughly 
100 mA at + 5 V and 70 mA at —5 V. 


Calibration 

Apply a sine wave of about 1 kHz at 
50 mVpp to input A and set PI to maxi- 
mum sensitivity (make sure the pot is 
properly connected; when it is at maxi- 
mum the wiper must be at a voltage of 
' —5 V d.c.). Trim preset P2 so that the 
meter shows the frequency stably on the 
display. Reduce the amplitude of the input 


3.28 elektor india march 1985 













signal and try to set P2 so that the fre- 
quency is still measured stably. Repeat 
this procedure a few times until the 
optimal setting of P2 is found. The meter 
must work properly from at least 30 mVpp. 
If this is not the case, even at larger input 
voltage levels, check the connections of 
Tl. 

Next we apply a signal of about 20 MHz at 
50 mVpp to the C input (after choosing 
input C, less than 100 MHz from the 
menu). Assuming that IC7 is used, turn 
preset P3 completely to the right. Set the 
HF input to maximum sensitivity with 
preset P4. Reduce the input signal ampli- 
tude progressively until a setting is found 
that still gives a stable read-out. Use the 
menu buttons to choose the C input at 
greater than 100 MHz but apply no signal 
to the input. Turn P3 slowly towards the 
left and stop when the trigger LED starts 
to flash. The SP8755 is now oscillating, 
which is quite normal for this sort of 
divider when set to maximum sensitivity 
in the absence of a signal. Turn P3 slightly 
back so that the LED no longer flashes. 

A second piece of metal can now be 
made (with the same shape as the 
bracket) to provide a screen for the com- 
ponent side of the board. Solder the two 
pieces of metal together at the top after 
covering the inside of the second piece 
with insulating tape or something similar 
to prevent short circuits. Some mounting 
bosses in the top of the case must be 
removed to enable it to close. Make sure 
that there are enough holes in the top and 
bottom of the case to ensure sufficient 
ventilation but do not make them so large 
that the 220 V connections are exposed. 


Operating instructions 

Perhaps 'operating instructions’ is a bit of 
a misnomer for this section as the fre- 
quency counter is quite simple to use. 
What we had in mind is more of an 
introduction to the few controls it does 
have. 

The menu of the meter is reproduced in 
figure 6 as this is the base from which we 
always work. In the vast majority of cases 
the user will know what sort of signal is 
being measured and will therefore know 
whether to feed it to input A, B or C. 
When the meter is switched on it selects 
the 'frequency' position and input A. To 
choose another function press the menu 
button. First we get the main choices: fre- 
quency, period time, pulse time or pulse 
count (event counter). Choose one by 
pressing the ‘YES’ and ‘NO’ buttons as 
appropriate. The next selection to be 
made concerns the input. If a frequency 
or period measurement is already chosen 
inputs A, B and C are all available, but for 
pulse time and event counter only inputs 
A or B may be selected. If input C is 
chosen there is a further choice to decide 
if the prescaler is needed (above 
100 MHz). With frequency or period 
measurements there follows a choice of 6 



GHz input stage 


or 7 digit accuracy. For 6 digits the 
measuring time is less than 0.2 s, and if 7 
digit accuracy is chosen (which means a 
ten-fold improvement) the measuring time 
is ten times as large so it is less than 2 s. 
When pulse time measurement is 
selected the meter must still be told 
whether the ‘0’ or T time is to be 
measured. This just leaves the choice of 
positive or negative slope in the event 
count mode. This selection simply deter- 
mines whether the counter reacts to rising 
(positive) or falling (negative) edges of the 
input signal. 

That covers all the frequency counter’s 
functions but there are still two buttons 
'hat have not been dealt with. The ‘LAST’ 
button is used to jump back (as figure 6 
indicates). If an error is made during 
selection the LAST button can be pressed 
to move one step backwards. The function 
of the ‘HOLD/RESET’ button is not indi- 
cated in the menu. When this button is 
pressed once the read-out is frozen and 
no more readings will be made. The indi- 
cator LED above the button then lights. 
Pressing HOLD/RESET again sets the dis- 
play to zero and the meter starts counting 
again. 

As we said at the beginning of this section 
the frequency counter is very simple to 
use because it lends the user a helping 
hand. That entirely justifies the brevity of 
these ‘operating instructions’ as you will 
soon see when you start using the meter. 

H 


elektor mdia march 1985 3.29 



microphone preamplifier 


Microphone extension cables are typical sources of noise. Signal 
losses caused by such cables are normally compensated by an input 
preamplifier, but this also amplifies the noise generated in the cable, 
as well as any random noise picked up by the cable. For those cases 
where the extension cable exceeds, say, one or two metres, it seems 
sensible to amplify the microphone signal before and after the cable. 
The advantages of this suggested configuration are a much improved 
signal-to-noise ratio and more effective hum suppression. 


microphone 

preamplifier 


with 

symmetrical 
signal transfer 


The proposed circuit consists of two parts, 
of which one is inserted between the 
microphone and cable and the second 
terminates the cable at the other end. A 
block schematic of the set-up is shown in 
figure 1. The signal from the microphone 
is therefore amplified by 20 dB before any 
cable-induced noise or hum is superim- 
posed onto it. 

The two amplifiers are connected by a 
two-core individually screened cable, 
which further reduces hum and noise 
pick-up. Note that the required power for 
the first part of the circuit (A) is supplied 
via the cable; this keeps the weight at the 
microphone as low as possible. 

The second part of the circuit (B) 
amplifies the signal by a further 12 dB to 
make it suitable for driving the power 
amplifier via the TAPE, TUNER, or AUX 
terminals. 

Normally, one of the terminals of the 
microphone inset is connected to earth 
while the other is used as the signal out- 
put. This would also have been possible 
with the output of the 20 dB amplifier: one 
of the cable conductors would then have 
served as signal line and the other as the 
earth line. We have, however, opted for a 
different set-up: one of the amplifier out- 
puts, 1 (+), carries the normal signal; the 
other, 2 (— ), the phase-inverted signal. 

If nothing further were done, the output of 
the 12 dB amplifier would be zero, 
because the two anti-phase signals would 
cancel one another. The phase-inverted 
signal is, therefore, inverted again and 


added to the signal on the other line. All 
this is clearly illustrated in figure 1. 

Why go to all this trouble? Because the 
noise signals on the second line are also 
inverted in the 12 dB amplifier and added 
to the noise signals on the first: as they 
are in anti phase, they cancel each other 
to a large extent! 

The circuit diagram 

Parts A and B of the block diagram are 
easily recognized in the circuit diagram in 
figure 2. The 20 dB amplifier is built 
around transistors T1 and T2; the exten- 
sion cable is connected between 1 and 2 
and T and 2’ respectively; the 12 dB 
amplifier comprises transistors T3 . . .T5 
and associated components. 

Transistor T1 amplifies the microphone 
signal about tenfold. The gain factor is pri- 
marily dependent upon the ratio R6:R5. If, 
for instance, the microphone signal is 
around 10 mV, the collector voltage of T1 
will be about 100 mV. 

Transistor T2 applies the signal at the col- 
lector of T1 to the extension cable twice: 
normal to 1 and phase-inverted to 2. Note 
that the collector and emitter resistors of 
T2 are located in the 12 dB amplifier (R8 
and R9 respectively). As the two resistors 
are identical, the signals at the collector 
and emitter have the same level, but are 
opposed in phase. 

RC network R7/C3 is a low-pass filter 
which prevents any signal feedback to the 
input stages. 


Figure 1. Block schematic 
of the two parts of the 
preamplifier connected by 
a two-core individually 
screened cable. 



3.30 elektor mdia march 1985 


microphone preamplifier 





Transistors T3 and T4 serve to invert the 
phase of the signals on one of the lines, 
and to add the two signals together: the 
latter is effected by common emitter 
resistor Rll. 

The signal at the collector of T3 is applied 
to T5 which amplifies it fourfold. The 
amplifier signal is then applied to the out- 
put via a high-pass filter which prevents 
any direct voltage reaching the output. 
Resistor R2 serves to match the micro- 
phone output impedance to the transistor 
input impedance. You will remember that 
optimum performance ensues if the input 
impedance of the amplifier is equal to, or 
somewhat greater than, the output 
impedance of the microphone. In the cir- 
cuit in figure 2, the input impedance is 
determined primarily by the resulting 
value of R3 and R4 (which are effectively 
in parallel): as shown this value amounts to 
57 kQ. If this value is too different from the 
microphone impedance, it may be 
lowered by R2. For instance, if, in the 
example given above, R2 is given a value 
of 100 kQ, the input impedance of the 
amplifier would reduce to 36 kS. 


Construction 

The printed circuit board for the two 
amplifiers is shown in figure 3: this should 
be cut before assembly. Ideally, the part 
for the 20 dB amplifier should fit in the 
microphone housing, but in many cases 
this may not be possible (the comers of 
the board may, of course, be rounded with 
a file to make it easier to fit, but be care- 
ful not to damage the tracks!). Otherwise 
the amplifier should be housed in as small 
a metal box as possible and mounted 
close to the microphone Ideally this 
should be done by means of a plug and 
socket. In any case, make sure that the 
earth connections between the units and 
the screen of the cable are sound. 

The part for the 12 dB amplifier will, we 
feel sure, give no trouble in being fitted 
inside the power amplifier or mixer unit 
cases. It will normally also be possible to 
tap the required supply voltage in these 
units. H 


Figure 2. The circuit 
diagram of the 
preamplifier: this has 
been kept as simple as 
possible to enable 
particularly the 20 dB 
amplifier to be built into 
the microphone housing. 


Figure 3. The printed cir- 
cuit board for the 
preamplifier: note that 
this should be cut before 
assembly. 

Parts list 

Resistors: 

R1,R10,R12 = 1k8 
R2 = see text 
R3 = 390 k 
R4 = 68 k 
R5.R8.R9 - 1 k 
R6 = 10 k 
R7 =*= 15 k 
R 1 1 = 2k2 
R13 = 1k2 
R14 = 4k7 
R15 = 100 k 

Capacitors: 

C1,C4 = 10 p/16 V 

C2 = 1 n 

C3 = 100 p/10 V 

Semiconductors: 

T1.T2.T3 = BC549C or 
BC550C 

T4.T5 = BC 559C or 
BC560C 

Miscellaneous: 

SI = SPST switch 
2-core ( individually screened 
audio cable as required 
PCB 85009 


elekior india march 1985 3.31 





remote model control 
with a microcomputer 


Photo 1. For our tests and 
evaluation we used a 
Multiplex Royal me'. The 
transmitter can be 
switched between pem 
and pdm. The basic ver- 
sion has four channels 
and may be extended to 
up to fourteen functions. 
Adaption to the control 
characteristic of different 
models is possible by 
means of a 'sort module' 
(ROM). 



PCM instead of 
PDM 


remote model control 
with a microcomputer 

As was to be expected, it has not taken very long after the 
microcomputer began to be used in good-quality portable radio 
receivers for it to encroach upon remote model control systems. 
Remote control using pem (pulse code modulation) is a typical 
microcomputer application: a long overdue innovation. 


In binary pulse code modulation only cer- 
tain discrete values are allowed for the 
modulating signals. The modulating signal 
is sampled and arty sample falling within a 
certain range of values is given a discrete 
value. Each value is assigned a pattern of 
pulses and the signal is transmitted by 
means of this pattern (code). In remote 
control, the transmitted signal cor- 
responds to the position of a joystick. 

PDM — the conventional method 

Pulse-duration modulation (pdm) is a form 
of pulse-time modulation (ptm) in which 
the time of occurrence of the leading 
edge or trailing edge is varied from its 
unmodulated position. In remote control 
systems, the joystick potentiometer is 
made part of a monostable multivibrator 


(MMV) circuit. With the joystick in its 
centre position, the MMV generates 
pulses of 1.5 ms duration; at the two end 
positions of the control column, pulses of 
1 ms and 2ms respectively are produced. 
In multi-channel equipment (each channel) 
requires a joystick potentiometer), the 
MMVs operate sequentially so that in each 
run a pulse train is generated. After each 
run (or cycle), the transmitter arranges an 
interval of 10 ms before the next cycle can 
begin. This is how the modulating signal 
in figure 1 is produced. The interval is 
needed for the synchronization of the 
decoder in the receiver: it ‘warns’ the 
decoder that a new cycle is about to start. 
The decoder then arranges for the incom- 
ing pulses to be directed to the appropri- 
ate servos: the first pulse to servo 1, the 
second to servo 2, and so on. A regulator 


3.32 


elektor india march 1985 



circuit in the servos ensures that the servo 
is driven in accordance with the duration 
of the received pulse. 

PCM — the modern method 

Like other digital computers, the micro 
cannot work with the analog values (of 
current, voltage, resistance) emanating 
from the joystick(s); it needs binary digits, 
bits, at one of its input ports. The proven 
means of converting a continuously vary- 
ing (analog) signal into a series of bits is a 
digitizer also called analog/digital con- 
verter. 

Unfortunately, analog/digital converters are 
relatively expensive, so it is not feasible to 
connect one to each joystick poten- 
tiometer. As in pdm systems, the signals 
at the various potentiometers in the con- 
trol levers are passed sequentially to the 
analog/digital converter: this is called 
multiplexing. As each scanning cycle 
takes several milliseconds, there are no 
speed problems associated with the 
digitizer. 

If you already have pdm equipment, you 
do not need an analog /digital converter 
because the pdm signal (figure 1) is 
already digital. This digital signal is then 
fed to one of the serial ports of the micro- 
processor and the micro then simply 
evaluates the pulse durations. The counter 


position at the end of the pulse is the 
binary value for that particular poten- 
tiometer. This solution is suitable for 
equipment that can be switched between 
pdm and pcm so that the new transmitter 
can still work with existing pdm receivers. 
An 8-bit analog/digital converter provides 
up to 2 8 (= 256) binary nummers. This 
means that, functionally, the joystick 
potentiometer may be compared to a 
rotary switch with 256 positions, so that 
the relevant servo may assume 256 differ- 
ent positions. This is illustrated in figure 2. 
The graduated disc above the servo shows 
the relation between the servo position 
and the 8-bit binary word (= byte). The 
rectangular pulses illustrate a portion of 
the received pcm signal. A servo fitted 
with a step motor and relevant control 
could be driven direct by the byte, but 
such servos are (not yet) available in the 
retail trade. To drive conventional servos, 
the pcm decoder in the receiver must 
convert the pcm signal into control pulses 
of variable width (pdm). 

Circuit technique 

We shall use the circuit diagrams of 
Microprop’s pcm equipment as an example: 
the transmitter diagram is shown in figu- 
re 3, that of the receiver in figure 4. 

Starting with the transmitter, at the left in 



remote model control 
with a microcomputer 


Figure 1. Conventional 
digital pdm system. The 
servo position is deter- 
mined by a pulse of 
definite duration (1. . .2 
ms). 


elektor mdia march 1985 3.33 


remote model control 
with a microcomputer 


Figure 2. With pulse code 
modulation (pcm). the 
servo positions are 
divided into discrete 
steps. Each servo position 
is determined by a binary 
number consisting of 8 or 
9 bits. 


Figure 3. Circuit diagram 
of the Microprop pcm 
transmitter in which the 
different functional 
blocks are clearly iden- 
tifiable. At the left, the 
controls (joysticks, and so 
on), then the analog/ 
digital converter, and next 
the single-chip microcom- 
puter. At the top right the 
supply regulation and at 
the bottom right the bat 
tery monitor circuit with 
alarm buzzer. 


2 


1 2 3 4 S 6 

10 0 10 1 



figure 3 are the joysticks, slide poten- 
tiometers, presets, and channel switches. 
Between these components and the 
analog /digital converter is a 64-way 
connector which enables the insertion of 
a special module. Such a module contains 
a variety of potentiometers and a number 
of operational amplifiers and makes it 
possible to preset, for instance, the trim or 
steering controls of a particular model, or 
the combining of several control functions 
(called mixing). It is, for example, possible 
in the transmitter to mix electronically the 


height and rudder functions of the tail 
unit. The module also enables the 
modification of the control characteristic, 
for instance, from linear to exponential. 

The voltages from the control elements 
are applied to the eight inputs of IC3 via 
operational amplifiers (opamps). This cir- 
cuit contains the multiplexer and the 
analog/digital converter. The clock for the 
switching of the inputs and of the 
analog/digital converter is provided by 
microprocessor IC6. This single-chip 
device, a CMOS version of Motorola's 
6805, is also fed with the data from the 
digitizer via the data bus. It is also poss- 
ible to connect eight switches to this bus 
via the nautics socket; the micro will then 
scan the signals from these switches 
instead of the information on channel 5 
when switch S5 is closed. The micro pro- 
cesses the 8-bit data words into a serial 
(pcm) signal on pin 5, which is a combi- 
nation of the data words and additional 
test or synchronization bits. This signal is 
then fed to the input connector of the h.f. 
module (which contains the transmitter 
proper) via a buffer type BC 239. 

The RESET switch connected to pin 17 of 
the micro does not serve to reset the 
transmitter computer, but to switch off for 
10 seconds the low-voltage warning func- 
tion in the receiver! 

A simple voltage regulator (right-hand top) 
consisting of an opamp, zener diode, and 
a regulating transistor provides the circuit 
with + 5 V. Three further opamps con- 
tained in IC7 (bottom right) form the low- 
voltage warning circuit for the transmitter. 
Total current consumption amounts to 


3 



3.34 Hiektor india march 1985 




about 150 mA — without the h.f. module 
around 50 mA. 

The decoder board of the receiver 
(figure 4) contains a microprocessor ident- 
ical to that in the transmitter, but different- 
ly programmed, of course. 

The r.f. part of the receiver, contained on 
a separate board, complies with the nor- 
mal standard requirements for fsk (fre- 
quency-shift keying) remote control sys- 
tems: no r.f. amplifier, an S042P as mixer/ 
oscillator (quartz controlled), a 455 kHz 
selective band-pass filter, and an S041P as 


limiter/demodulator/amplifier. The signal 
from the r.f. section is first amplified in 
two of the four opamps in IC3 and then 
reshaped to rectangular pulses before it is 
fed to a bus input (pin 6) of the microcom- 
puter. The other seven inputs of the micro 
are not used and are connected to +5 V. 
As in the transmitter, a simple power-on- 
reset circuit is connected to pin 1. All 
further processes are carried out within 
the chip under the control of the software; 
the outputs of the micro are taken direct 
to the servo connectors. The servos are 


Photo 2. The pcm coding 
module in the Webra 
transmitter, in which 
another 80C48 carries out 
almost all the work. 

There is no separate 
analog/digital converter 
and other peripheral com- 
ponents have also been 
kept to a minimum. 


Figure 4. Circuit diagram 
of the Microprop pcm 
receiver which is built 
onto two PCBs. The top 
part shows the f.m. 
receiver, a simple 
superhet with mixer 1C, 
ceramic filter, and 
demodulator 1C. The 
lower board contains the 
decoder and the single- 
chip microcomputer 
which is of the same type 
as that in the transmitter 
but. of course, has been 
programmed differently. 
The micro's outputs drive 
up to eight servos direct 
by means of variable 
width (1. . .2 ms) pulses. 
Two opamps amplify the 
received pcm signal and 
convert this into rec- 
tangular pulses for driv- 
ing the microcomputer. 
IC4 doubles the battery 
voltage which is then 
stabilized at 5 V by a 
further opamp. The 
fourth opamp contained 
in IC3 monitors the bat- 
tery voltage. 



elektor india march 1985 3.35 





remote model control 
with a microcomputer 


Photo 3. Webra's receiver 
and pcm decoder. The 
receiver is based on the 
well-known S041P and 
S042P. The decoder cir- 
cuit consists basically of 
OKI's CMOS processor 
type 80C48, which is loca- 
ted under the 3.88 MHz 
quartz crystal on the 
decoder board. The sec- 
ond chip is a four stage 
comparator type LM 339. 
The eight servos are con- 
nected direct to the 
80C48. 



controlled by variable-width pulses. 

The 4.8 V receiver battery (four NiCd 
cells) is connected to terminal B. To 
ensure a supply of 5 V, the battery voltage 
is doubled by IC4 and then stabilized at 
+ 5 V by IC3 and a type BC 308 transistor. 
Zener diode ZN458 provides a reference 
voltage of 2.45 V for the voltage regulator 
and for a fourth opamp which monitors 
the battery voltage. As soon as that voltage 
drops below 4.5 V, the opamp makes pin 8 
of the micro logic 0 which starts the warn- 
ing procedure. Current consumption of 
the receiver proper is about 35 mA while 
each servo draws a quiescent current of 
around 10 mA. Thanks to the voltage 
doubling, the receiver continues to func- 
tion satisfactorily until the battery voltage 
drops to about 3.4 V. 

Signal processing and 
transmission 

The transmitter micro composes from the 
byte from the analog /digital converter a 
serial signal that also contains any test or 
synchronization bits. In some equipment a 
channel address is also added. In the 
example given in figure 5, each 8-bit data 


block is followed by the relevant channel 
number (3 bits for channels 1. . .8), a par- 
ity bit, a stop bit, and a sync pulse. After 
one cycle (eight blocks for channels 
1 ... 8) has run, the next cycle starts with 
channel 1 again. If you add all the bits 
together, you will find that there are 104 
bits per cycle, excluding the sync pulses. 
In most conventional equipment the 
cycles last 20 ms so that with eight chan- 
nels the transmission rate becomes more 
than 5000 bits per second. This means that 
with a channel spacing of 10 kHz, the 
transmitted r.f. bandwidth becomes too 
wide. There are two possible solutions to 
this: reduce the number of bits or 
increase the cycle time. Depending on the 
manufacturer, you will find the following 
solutions: 

■ priority channels — only three or four 
channels are transmitted each cycle; 

the others less often, for instance, each 
second or fourth cycle; 

■ priority principle — here the transmitter 
micro composes the cycle in order of 

priority of the data blocks. For that pur- 
pose it needs to be first established in 
which channels something is changing. 
Such active channels (joystick movement) 


Figure 5. Composition of 
a pcm transmission cycle. 
Each cycle here consists 
of eight blocks. Each 
block contains the infor- 
mation for one channel in 
8 bits, followed by the 
channel address in 3 bits, 
a parity bit. and a stop 
bit. Each block is fol- 
lowed by a synchron- 
ization) pulse. 


1 0 0 0 1 0 0 1 0 0 0 1 


00011101 


10 111 


z t 
«z In 
o < < 

O I CL 

< u 


- 

tick- 


et 2 
o < 
9 1 

< o 


S z 

S 4 
9 1 

< o 


3.36 elektor india march 1985 



4 



are transmitted more frequently and the 
others only every second or fourth cycle. 

■ longer cycle time — all channels are 
transmitted in fixed order; the cycle fre- 
quency drops therefore to about 20 Hz. 
Recently, Japanese equipment OR and 
Futaba) has become available that uses a 
different solution: they operate with 9-bit 
data words, do not use priority channels 
or the priority principle, and yet operate 
at 50 full cycles per second. The Japanese 
have apparently developed a time-saving 
coding system! 

Signal transmission from transmitter to 
receiver takes place as in conventional 
pdm equipment by frequency-shift keying 
(fsk); most manufacturers use the same r.f. 
module. The pcm signal from the micro is 
filtered to round the edges and is then 
used to modulate the carrier via a varicap. 
The receiver also uses the f.m. standard. 
Receivers with gain-controlled r.f. 
amplifiers are just beginning to become 
available. These are long overdue because, 
after all, receiver characteristics such as 
the signal-to-noise ratio, selectivity, and 
sensitivity are just as important here as in 
most other receivers and they are cer- 
tainly ripe for improvement. It should also 
be borne in mind that the microcomputer 
cannot improve the r.f. performance of the 
receiver, although it can detect trans- 
mission errors by means of the test bits. 
Depending on the manufacturer, the micro 
tests single data blocks or whole cycles at 
parity. At least one manufacturer 
(Microprop) relies on a cyclic redundancy 
check (CRC). If data are suspected of con- 
taining errors, they are not passed on to 
the servos, although error correction is not 
available. As long as correct information 


does not become available, the servos 
retain the status quo. None the less, after 
0.5 .. . 1.5 seconds (depending on manufac- 
turer), the micro takes emergency 
measures. 


Photo 4. The inside of the 
'Royal me' transmitter is 
quite neat. The r.f. part 
has been designed as a 
plug-in module. 


Action in emergency 

All pcm receivers have a more or less 
‘clever’ fail-safe program. In the simplest 
case, the servos remain in the last cor- 
rectly received position. An alternative is 
to slow down the engine or set the servos 
to neutral. In most receivers it is possible 
to choose between these alternatives. 
Additionally, some Japanese equipment 
and also the Austrian Webra pcm unit 
offer the model aviator the possibility of 
establishing his own emergency measures 
and store these in the transmitter micro. 
When the system is switched on, the 
measures are then cyclically radiated, 
stored in the receiver, and acted upon in 
an emergency. 

The receiver also reacts to the dropping 
of the battery voltage below a certain level 
as to an emergency. The most rabid com- 
puter reaction is the slowing down of the 
engine or the application of the brake 
flaps in gliders. Rather less drastic 
measures are also possible; with these it 
is generally possible to determine yourself 
which function to trip at low battery 
voltage. Futaba and Webra allow the 
model aviator to switch off the tripped 
low-voltage warning function at the 
transmitter and to land the model without 
inhibition of any control function. 

PCM in practice 

PCM equipment with 8-bit resolution 
moves its servos in small but clearly 


elektor india march 1985 3.37 


remote model control 
with a microcomputer 


Photo 5. Measurements 
in the receiver: at the top 
the output signal to a 
servo, a variable-width 
pulse which is fed to the 
servo every 25 ms. Below, 
the pcm signal at the 
input to the microcompu- 
ter in the pcm decoder: a 
constant stream of bits at 
the rate of 2500 bps (bits 
per second). 


Photo 6. At the top the 
r.f. spectrum of the Mul- 
tiplex Royal me' transmit- 
ter switched to pcm 
operation. As a compari- 
son, the spectrum of the 
same transmitter 
switched to pdm oper- 
ation is given below. The 
horizontal scale is 2 kHz 
per division and the verti- 
cal is 10 dB per division. 


Photo 7. The modulation 
signal in pdm operation. 





Different assessments by the manufac- 
turers bring about different methods of 
limiting the transmitter bandwidth. Equip- 
ment with ‘faster’ priority channels, for 
instance, is not quite suitable for appli- 
cations where mixing functions are 
important (as in expensive gliders and 
helicopters). Practical competition pilots 
with their equally practical models will 
detect in Multiplex and Webra equipment 
a small, but noticeable delay in response 
which can be traced back to the reduced 
cycle frequency. 

What is undoubtedly a very positive factor 
in a pcm system is its facility for suppress- 
ing interference. The total absence of the 
dreaded ‘servo wobble’ gives pilots 
greater confidence in critical situations as, 
for instance, in low-level fly-overs at high 
speed. Also, there is no longer the danger 
of a near-by operating transmitter upset- 
ting things when the model is taxiing. 
There is also a negative aspect in that 
when the limit of the operating range is 
reached, this is no longer indicated by 
‘wobble’. A pcm-controlled model reacts 
either correctly or not at all. There is no 
‘grey area’. 

Confidence is further strengthened by the 
low-voltage warning circuit, although a 
drastic slowing down of the engine as a 
warning signal can lead to awkward situ- 
ations. In this respect, warning signals that 
the pilot can establish himself and which 
can be disabled via the transmitter are 
much to be preferred. 

Many are the arguments as to the sense 
and nonsense of the various fail-safe pro- 
grammes, although most experts do not 
rate the probability of a complete rescue 
very high. When the transmitter fails, or 
there are very strong interfering signals, 
even pcm-controlled models can crash, 
although they do so more neatly than 
others: in tight bearing with definite con- 
trol settings and slowed down engine! 


7 


• i l L ■ i ^ • |-B * 

_ m \x\,w 

ninth 

iiiii. 

. ..mill*...- - 



discernible steps, accompanied by a soft, 
purring noise. The quantization error of 0.4 
per cent is of the same order as the pos- 
itioning accuracy of the best servos 
(under no-load conditions). With 9-bit res- 
olution, the error can no longer be deter- 
mined: the servos run just as smoothly as 
with pdm. 


Summary 

The most laudable aspect of pcm systems 
is the inherent interference suppression 
which completely obviates uncontrolled 
servo flounder. Battery monitoring in the 
receiver is also a welcome plus point. The 
various fail-safe programs are of con- 
siderable technical interest, but their prac- 
ticability is as yet questionable. 

In all fairness, it should be said that cur- 
rent first-class conventional equipment has 
reached a high degree of sophistication 
and is in practice wholly adequate. But 
pcm is more up-to-date and, when its 
price comes down to that of conventional 
equipment, offers a little more for your 
money. It should not take all that long 
before the microcomputer will also be 
available in inexpensive remote control 
equipment. H 


3.38 


ulektor india march 1985 


d.i.y. connector 


i 


I 

I 


It is very frustrating to be unable to complete a project or use some 
equipment for lack of the right connector. This happens to everybody 
including Elektor designers in spite of the great variety of connectors 
we have at our disposal. When we came up against this problem we 
felt the need to do something about it and rummaging around in a 
'junk' box found just what we needed. 

d.i.y. connector 


These days there are norms for virtually 
everything. The most common standards 
for connectors are DIN (German standards 
association) and the new SCART 
(described in the October 1984 issue of 
Elektor India). Even modern equipment 
does not always conform to norms, however, 
and this can cause problems. Difficulties 
can also arise if you cannot resist that 
‘bargain of the century’ but find that it has 
a completely unique type of connector. 
Provided this is a female connector there 
is a solution. This is what to do: 

■ Start by finding a suitable type of con- 
tact or pin to suit the size of the female 

connector and that will provide a good 
electrical contact when inserted. 

■ Cut a piece of perspex about 3 mm 
thick to almost the same dimensions as 

the connector. This should be made 
slightly oversize to ensure a tight fit. 

■ Drill guide holes in the appropriate 
positions using a bit that is about 

0.3 .. . 0.S mm smaller in diameter than the 
pins chosen. 

■ Place the perspex in a vice and close 
the jaws (but not too tightly). 

■ Push the pins into the perspex one at a 
time by heating them with a soldering 

iron. The guide holes will now melt to the 
right size. 

■ A pliers can be used to make slight 
adjustments to the positions of the pins 

if they are heated again. 

This technique, as you will have noted, 
can only be used to make male connec- 
tors. The result is shown in photo 1. If the 
connector to be made has a standard 
layout the matter is simplified somewhat: 




■ Take a length of prototyping board with 
holes spaced at 0.1 inch (2.54 mm) and 
solder the pins at the appropriate places. 
That is all there is to it! 

Note that there are European connectors 
(31-pin, for example) with the pins spaced 
at 2.5 instead of 2.54 mm. These two types 
are not compatible. 

Male connectors can be made for most 
types of female sockets. A few different 
types are shown in photo 2, to give some 
idea of the options. 


Two connectors for the price of 
one 

There is sometimes a need for a female 
edge connector with 2.54 mm spacing (as 
in the CPU card published in November 
1983). These are becoming more common 
but you may not have one when you need 
it. The answer might be found in your 
junk-box, in the form of a 34-way connec- 
tor salvaged from a ribbon cable. Two 
connectors, with up to 16 ways each, can 
be made from this. 

■ Cut the 34-way connector in half with a 
hacksaw or something similar. This 

renders the two centre connectors use- 
less. If necessary trim each half to the cor- 
rect number of ways. 

■ File the cut ends of each section. • 

■ Solder the wires of a multicore or rib- 
bon cable carefully to the appropriate 
pins. 

■ Spread a thin layer (about Vi to 1 mm) of 
two-component glue between the two 
rows of pins. 

■ Fix the cables in place with a few 
drops of two-component glue. Make 

sure the glue used does not attack the 
insulation on the wires. Apply several thin 
layers of glue until the ends of the wires 
are fully encased. When this is finished 
the result will be like the example shown 
in photo 3: a simple-to-make but virtually 
indestructible connector. H 


the art of 
making your 
own connectors 



elektor india march 1985 3.39 


programmable rhythm box 


convert your 
micro-computer 
into a 


Most personal computers on the market today have at least one 
'voice': a sound generator, in other words. The circuit here is 
something completely different. It is more a matter of computer 
control than microprocessor-generated sound. The computer controls 
eight generators, each of which provides the sound of a particular 
percussion instrument. The ZX81 is used as an example to show that 
the computer controlling the drum box does not have to be very 
sophisticated or have a large memory in order to perform a useful 
task. 


programmable rhythm box 


. « - •> . j i -i 

' 3~~' s 

- i - - t - 


4 l . t., «■; 1 

a ± 

~ | 

r r u 

v^3_ 

'jrrr 1 


H. de Lange 


Table 1. Each control data 
bit output by the micro- 
computer corresponds to 
the input of an instru- 
ment generator. Each 
generator is active when 
its input goes from '0' to 
T. 


Eight-bit micro-computers can be used to 
perform many tasks, even, as is the case 
here, play an electronic drum set. The 
procedure involves generating a 
sequence of data whose binary configur- 
ation (the order of Ts and ‘0’s) triggers the 
different noise generators in turn. The 
data determines the rhythm and tempo of 
the total sound generated. Each of the 
eight output data bits that make up the 
control word corresponds to the control 
input of one instrument. This is indicated 
by table 1. If the data word were 
0000 0001, for example, a single instru- 
ment, the bass drum in this case, is 
triggered. 

If the binary control word is 0000 0000 
nothing will be heard as no instrument is 
triggered. If the control word is 1111 1111 all 
the instruments will be triggered. The 
result in this latter case will be just a con- 
fusion of noise as no more than three or 
four instruments can be triggered at a 
time without introducing distortion. 

One essential part of the drum box is the 
eight noise generators, the other essential 
is the eight-bit control word provided by a 
computer. The word appears directly on 
the ZX81’s data bus but may have to travel 
via an output port (VIA, PIA, PIO, etc.) in 
the case of other computers. The program 
makes use of arrays, the number of 


Table 1. 



elements of which is determined 
beforehand using the DIM instruction in 
BASIC. The number of elements in the 
array decides the length of the rhythmic 
sequence that must be repeated. A simple 
POKE command is used to apply the data 
word to the noise generator circuits. The 
use of BASIC does not really limit the 
speed at which the rhythms are executed. 

The interface 

The interface between micro-computer 
and sound generator is shown in the cir- 
cuit diagram of figure 1. The section in 
the box is the adress decoding circuit 
that is specific to the ZX81 and has the 
Elektor bus pin configuration. The logic 
level of line A0 is not examined so the cir- 
cuit is active at both 3FE0HEX and 
3FE1HEX (16352 and 16353 respectively in 
decimal). The logic level output from N10 
when one of these addresses appears on 
the address bus is combined with the 
logic output of N12, whi ch is low when 
control lines MREQ and WR are both ‘O’. 

In this way Nil outputs the enable signal 
for the interface. 

In a 6502-based system the MREQ and 
WR signals are replaced by the single 
RAM R/W signal. The address decoding 
must be modified to suit the particular cir- 
cumstances by means of inverters 
Nl. . ,N5 and gates N7. . ,N9. 

The addressing signal output from Nil 
triggers monostable multivibrator N15/N16. 
This in tum controls an indicator LED via 
N13 and N14, the parallel combination of 
which satisfies the LED’s current require- 
ment. Every time the circuit is addressed 
(when at least one instrument is triggered) 
the LED lights. This gives a visual indi- 
cation of the tempo. The same enable 
signal also controls octal flip-flop IC5. 


3.40 elektor india march 1985 




programmable rhythm box 


When the CLK input detects a falling 
edge the 74LS374 passes the word present 
on the computer’s data bus to electronic 
switches ESI . . . ES8. If the data word is 
fed through a programmable output port 
in the computer the octal flip-flop is 
unnecessary and can be left out. 

Each of the eight data bits controls an 
electronic switch (via ICS). Experience 
has shown that using these switches 
(which might appear superfluous) is an 
effective way of reducing the intermodu- 
lation between the various instruments. In 
addition to this the high impedance of 
these switches when they are open effec- 
tively suppresses the sound generated by 
the instruments. What it all adds up to is 
that the switches improve the circuit’s 
signal to noise ratio (in this case we could 
call it the noise to silence ratio). 

The width of the control pulse, as we will 
see shortly, effects the sound of some of 
the instruments. The instruments are con- 
trolled by the signals BD (bass drum), CD 
(conga drum), HB (high bongo), LB flow 
bongo), LC (long cymbal), CL (claves), MR 
(maracas) and SD (snare drum). 


The generators 

Three different types of noise generator 
are shown in the diagram of figure 2. They 
provide: 

■ muffled oscillation at a given frequency 

■ filtered (or coloured) white noise 

■ mixture of filtered white noise and 
muffled oscillation. 

The muffled oscillation is produced by 
double-T oscillators triggered by the con- 
trol pulses. The loop gain of these 
oscillators, each of which is based on a 
NAND gate, is controlled so that it is insuf- 
ficient for oscillation to continue. The 
degree of muffling therefore varies with 
the gain. The frequency of each of this 
type of oscillator is determined by the 
values of capacitors C2, C3 and C5 in 
each case. The output amplitude of every 
module is fixed by the value of resistor 
R8. The gain, and consequently the 
degree of muffling, can be changed by 
means of preset PI. 

The source of the white noise, T2, feeds 
the filtering circuit for the cymbal sound 
via C8. The actual filtering is carried out 


Figure 1. An octal TTL 
flip-flop is used here to 
latch the data input from 
the computer. The 
address decoding will 
have to be changed if any 
micro computer other 
than the ZX81 is used 
with this rhythm box. 
Note that the pin 
designations correspond 
to the Elektor bus. 


Elektor-Bus N1 . . . N6 = IC1 = 74LS05 



elektor india march 1985 3.41 










programmable rhythm box | £ 
5V 


T1.T3.T4 = BC 108B 
T2 = BC 140 



330o| R18 

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C17 05 R23 

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R22M ^ 2X 

M D4T1N4148 



N1 . . . N4 = IC1 = 4011 
N5 = %IC2 = 4011 


.Cl Ci r 3 01 

WlhOEFW 

4n7 1N4148 




1N4148 

■ 

HIGH BONGO 


HB Cl R3 01 

33" 1N4148 


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Figure 2. The actual 
rhythm box consists of 
five double-T oscillators 
(N1. . .N5) r each of whose 
muffled frequency has 
the characteristics of the 
instrument that is to be 
imitated, a white noise 
generator (T2) and two 
colouring networks for 
this white noise in the 
collector circuit of T3 and 
T4. The signals are 
summed at point B and 
the wiper of P4. 



i ivT 

R7 

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C5 





elektor India march 1985 












3 


programmable rhythm box 


12 V 



by L2 in parallel with R25, the combination 
of which tends to amplify high fre- 
quencies. Depending on whether the con- 
trol pulse is applied to the LC or SC input 
the cymbal sound will be long or short. 
The attack (rise) will always be very sharp 
and the decay (fall) will be long or even 
longer. The maracas sound is also 
generated using the same filtering circuit 
but the control signal (MR) is distorted so 
that the attack is progressive as this sort of 
instrument demands. The snare drum 
sound is achieved using an oscillator (the 
high bongo’s as it happens) and a noise 
filter. The SD control pulse is shaped by 
the circuit around Tl. The white noise is 
coloured by means of R13, LI and C9. The 
pulse is also fed to input A of the high 
bongo oscillator via D2, which prevents 
the HB signal from activating the snare 
drum’s noise circuit. 

The amplitude of the white noise signal 
applied to the filters is fixed once and for 
all with preset P2. The amplitude of the 
noise signal that figures in the snare 
drum's sound is determined by preset P3. 
The final mix of muffled oscillations and 
white-noise-based sounds is made by the 
wiper of preset P4. This, in fact, sets the 
input level of op-amp IC3. The output 
level can be varied with pot P6, while pot 
P5 allows the tone (actually the attenuation 
of the high frequencies) to be corrected. 

The power supply 

A suitable power supply for the rhythm 
box is shown in figure 3. If the controlling 
micro-computer can supply the necessary 
voltage and current (at least 100 raA at 
12 V) the power can be tapped directly 
from it and the circuit of figure 3 can be 
omitted. 


Two different printed circuit board 
designs are used for this drum box. The 
main board is shown in figure 4, and 
figure 5 shows the design that can be 
used for each of the five ‘instrumental’ 
modules. The photograph shows how the 
six boards fit together. The final result is a 
compact unit that can easily be accessed. 
The signal output by the rhythm box is not 
yet audible, of course. It must be 
amplified and fed to one or more 
loudspeakers. A telephone amplifier is 
quite sufficient for test purposes but will 
not reproduce all the sounds faithfully. Be 
careful about going to the opposite 
extreme however. If you feed the signal 
from the drum box into your hi-fi system 
keep the volume low. Even muffled the 
oscillations pack quite a punch. 

The software 

As yet the rhythm box cannot do anything. 
Without control signals the generators 
remain mute. The duration of the control 
pulses has no effect on the oscillators but 
the noise generators, on the other hand, 
do remain active as long as the corre- 
sponding control line is high. 

The program of table 2 allows eight 
‘classical’ rhythms to be generated. Each 
of these has a corresponding table, seven 
of which consist of 16 elements (the 
quavers making up two bars in four-four 
time). The table for the waltz, which is in 
three-four time, has only 6 elements. Each 
of these elements, A (C), is a control data 
word whose binary configuration activates 
one or more of the instruments. 

The control word is reset to zero regularly 
in a FOR-NEXT loop (E), the duration of 
which also decides the tempo. Line 440 
causes the FOR-NEXT loop (C) to repeat 


Figure 3. If the micro- 
computer cannot provide 
the power supply for the 
drum box the circuit 
shown here can be fitted 
to the printed circuit 
board shown in figure 4. 
Make sure voltage regu- 
lator IC4 is fitted with the 
right polarity. 


elektor india march 1985 3.43 




programmable rhythm box 



Figure 4. This printed cir- 
cuit board (which is not 
available from Elektor) 
can be used to make the 
rhythm box. The pin-out 
of IC4 used on the board 
is different from that of 
current 7812s: the blow- 
up shows the way this 
should be mounted. 

Figure 5. The components 
comprised in the double-T 
oscillators fit onto five 
printed circuit boards like 
this one. except for the 
NAND gates in IC1 and 
IC2 on the main board. 
Like the main board this 
one is not available from 
Elektor. The values of the 
components that are 
specific to each instru- 
ment are shown in figure 
2 . 


3.44 elektor mdia march 1985 



tableau 2 

2140 
2150 
2160 
2170 
2180 
2500 
2510 
2520 
2530 
2540 
2550 
2560 
2570 
2580 
2590 
2600 
2610 
2620 
2630 
2640 
2650 
2660 
2670 
2680 
3000 
3010 
3020 
3030 
3040 
3050 
3060 
3070 
3080 
3090 
3100 
3110 
3120 
3130 
3140 
3150 
3160 
3170 
3180 
3500 
3510 
3520 
3530 
3540 
3550 
3560 
3570 
3580 
3590 
3600 


programmable rhythm box 


LET A(13) = 33 

3610 

LET A(10» = 16 

LET A(14) = 0 

3620 

LET All 1 > = 144 

LET A(15) = 33 

3630 

LET AI121 = 147 

LET All 6) = 48 

3640 

LET AI131 = 19 

RETURN v 

3650 

LET AIM) = 16 

LET D = 16 

3660 

LET AI15) = 144 

DIM Af 161 

3670 

LET A(16) = 16 

LET A(1> = 164 

3680 

RETURN 

LET A(2) = 0 

4000 

LET D = 16 

LET A(3) = 164 

4010 

DIM AI16) 

LET A(41 = 0 

4020 

LET All) = 21 

LET A(5I = 2 

4030 

LET AI2) = 129 

LET A(6>=2 

4040 

LET AI3) = 1 

LET A(7> = 2 

4050 

LET AI4) = 144 

LET A(8) = 164 

4060 

LET A(5) = 5 

LET A(9) = 0 

4070 

LET AI6) = 129 

LET A(10) = 36 

4080 

LET A(7) = 21 

LET AI11) = 2 

4090 

LET AI8) = 129 

LET A( 12) = 36 

4100 

LET AI9) =5 

LET A(13) = 36 

4110 

LET AI10I = 129 

LET A< 14) =0 

4120 

LET A(11) = 17 

LET A{15) = 36 

4130 

LET AI12) = 129 

LET A(16) = 0 

4140 

LET AI13) =21 

RETURN 

4150 

LET AIM) = 129 

LET D = 16 

4160 

LET AI15) = 5 

DIM AI16) 

4170 

LET AI16I = 129 

LET All 1 = 146 

4180 

RETURN 

LET AI2) = 16 

4500 

LET D = 16 

LET AI31 = 48 

4510 

DIM AI16) 

LET AI41 = 145 

4520 

LET A( 1 ) = 6 

LET A(5) = 17 

4530 

LET AI2) = 49 

LET AI61 = 16 

4540 

LET AI3) =49 

LET A(7) = 176 

4550 

LET AI4) = 2 

LET A(81 = 16 

4560 

LET A(5) = 6 

LET A(9) - 17 

4570 

LET AI6) =49 

LET AI101 = 16 

4580 

LET AI7) = 4 

LET All 11 = 176 

4590 

LET AI8) = 49 

LET AI121 = 17 

4600 

LET AI9) = 6 

LET AI131 = 17 

4610 

LET AI10) = 49 

LET AI14I = 144 

4620 

LET All 1) = 49 

LET AI15I = 48 

4630 

LET AI12) =2 

LET AI161 = 16 

4640 

LET AI13) = 6 

RETURN 

4650 

LET AIM) = 49 

LET D= 16 

4660 

LET All 5) = 2 

DIM AI16) 

4670 

LET A( 16) =49 

LET All) = 19 

4680 

RETURN 

LET AI2) = 16 

9000 

CLS 

LET AI3) = 144 

9010 

PRINT "ANOTHER RHYTHM IY/NI' 

LET A(4) = 147 

9020 

INPUT F$ 

LET AI5) = 19 

9030 

PRINT F$ 

LET AI6) = 16 

9040 

IF F$= "Y" THEN GOTO 10 

LET A17) = 144 

LET AI8) = 16 

LET AI9) = 19 

9050 

STOP 


8 


10 PRINT "1 - BEAT 

20 PRINT ”3 = TANGO 

30 PRINT ”5 = BOSSANOVA 
40 PRINT "7 = BEGUINE 
50 PRINT 

60 PRINT "CHOOSE A RHYTHM" 

70 INPUT A 

80 PRINT A 

100 IF A> 8 THEN GOTO 60 
110 PRINT 

120 PRINT "CHOOSE A TEMPO (1-10)" 
130 INPUT B 

135 FAST 

140 PRINT B 

150 IF B 10 THEN GOTO 120 
160 IF A - 1 THEN GOSUB 1000 
170 IF A 2 THEN GOSUB 1500 
180 IF A = 3 THEN GOSUB 2000 
190 IF A 4 THEN GOSUB 2500 
200 IF A 5 THEN GOSUB 3000 
210 IF A 6 THEN GOSUB 3500 
220 IF A 7 THEN GOSUB 4000 
230 IF A 8 THEN GOSUB 4500 
240 CLS 

250 PRINT "TYPE 1 TO STOP" 

260 FOR C - 1 TO D 
270 POKE 16352, A(C) 

280 FOR E = 1 TO B 
290 POKE 16352,0 

300 IF INKEY $ "1" THEN GOTO 9000 
310 NEXT E 
320 NEXT C 
440 GOTO 260 
1000 LET D 16 
1010 DIM A(16) 

1020 LET A(1) = 65 
1030 LET A(2) =0 
1040 LET A(3) = 65 
1050 LET A(4) = 0 
1060 LET A(5> * 192 
1070 LET A(6) 0 
1080 LET A<7) 65 
1090 LET A(8) = 128 
1100 LET A(9) = 65 
1110 LET A(10) = 0 
1120 LET Alii) = 192 
1130 LET A( 12) = 1 
1140 LET A(13) = 64 
1150 LET AIM) = 128 
1160 LET A(15) = 65 
1165 LET A(16) - 64 
1170 RETURN 
1500 LET D = 6 
1510 DIM A(6) 


2 - WALTZ" 

4 = SAMBA" 

6 = ROCK AND ROLL" 
HABANERA" 


1520 

LET All) = 1 

1530 

LET AI2) = C 

1540 

LET AI3) - 128 

1550 

LET AI4) = 0 

1560 

LET AI5) = 128 

1570 

LET A<6) = 0 

1580 

RETURN 

2000 

LET D = 16 

2010 

DIM AI16) 

2020 

LET All) = 33 

2030 

LET AI2) = 0 

2040 

LET A(3> = 33 

2050 

LET AI4) =0 

2060 

LET AI5) - 33 

2070 

LET AI6) =0 

2080 

LET AI7) = 33 

2090 

LET AI8) - 48 

2100 

LET AI9) - 33 

2110 

LET A<10) = 0 

2120 

LET A( 11) — 33 

2130 

LET AI12) = 0 



endlessly. Line 300, on the other hand, 
provides an exit from the loop (to change 
the rhythm, for example) by pressing key 
T. 

Many BASIC compilers are familiar with 
the READ instruction which, in combi- 
nation with DATA, enables the program to 
have a more versatile and elegant struc- 
ture. If your computer has this command 
use it. 

The program shown here can only pro- 
vide very simple rhythms (two bars are all 
that is catered for). There is no reason not 
to extend the tables, however, to program 
more than two bars. With a bit of clever 
BASIC programming even breaks, 
fill-ins and other such features 
of style can be 
incorporated. M 


Table 2. The program 
given in this listing 
enables a ZX81 computer 
to control the rhythm box 
and make it generate 
eight basic rhythms with 
variable tempo. 


elektor indie march 1985 3.45 


EPROM selector 


Photograph. This shows 
the EPROM selector con 
nected to the 6502 CPU 
board featured in the 
December 1983 issue of 
Elektor India. 



EPROM selector 


four EPROMs in 
one address 
range 


byte 




hex 

dec. 

D1 

DO 

EPROM 

0 

0 

0 

0 

1 

1 

1 

0 

1 

2 

2 

2 

1 

0 

3 

3 

3 

1 

1 

4 

4 

4 

0 

0 

1 

5 

5 

0 

1 

2 

6 

6 

1 

0 

3 

7 

7 

1 

1 

4 

8 

8 

0 

0 

i 

9 

9 

0 

1 

2 

A 

10 

1 

0 

3 

B 

11 

1 

1 

4 

c 

12 

0 

0 

1 

D 

13 

0 

1 

2 

E 

14 

1 

0 

3 

F 

15 

1 

1 

4 


Table 1. The relation 
between the EPROM 
enabled and the relevant 
software command. 


The main memory of a microcomputer has a remarkable property: 
when you buy the computer, the memory seems very large, but as 
time goes on it shrinks and shrinks, a phenomenon caused, of 
course, by your programs getting longer and longer. . . The circuit 
suggested here expands the memory but only insofar as EPROMs are 
concerned. Basically, it is a simple but effective 'soft switch' which is 
suitable for all EPROMs in the 25XX and 27XX families. 


Every computer user knows that it is not 
possible to just write data into an EPROM, 
but that this must be programmed 
suitably. But what prevents the writing of a 
data word into an address range in which 
the EPROM is situated? After all, you can- 
not damage anything; at worst, the 
program ‘crashes’. With the present circuit 
you can intentionally write a data word in 
the EPROM address range: this will not 
affect the EPROM at all, but the decoding 
logic of the circuit will evaluate the infor- 
mation and on that basis select one of four 
EPROMs. That EPROM remains active until 
another one is selected by a fresh data 
word being written into the EPROM 
address range. All in all, a neat yet easily 
programmed solution to a frequent 
problem. 

The circuit 

Of the five sockets shown in figure 1, 
EPROM1 . . . EPROM4 are intended for the 


additional EPROMs, while the fifth accepts 
a DIL plug into which a length of flat rib- 
bon cable has been connected. The other 
end of the cable is also fitted with a DIL 
plug which is inserted into the original 
EPROM socket in the computer; that 
EPROM itself is plugged into one of the 
EPROM sockets 1. . .4. 

The circuit is contained on a PCB which, 
with few exceptions, connects the ident- 
ical pins of the sockets together. Connec- 
tions shown in brackets refer to 24-pin 
EPROMs, all others to 28-pin EPROMs. 
Exceptions are : 

■ terminals OE (output enable) of EPROM 
sockets 1 ... 4 are connected to the 

selection logic, which ensures that only 
one of the lines is logic 0 at any one time 
and therefore that only the selected 
EPROM is actuated; 

■ pins 20 and 22 of the master socket are 
connected to a wire bridge (pin 20 is 

also connected to pins 20 of the other 


3.46 elektor tndia march 1985 




EPROM selector 






Li 

1 A 

l>££ E 

3 

1 B 

IC2 2EN 

13 

2 B 

74 LS 139 yj" 

M 

2 A 

IEEE 


ici 

0 


£L 


5 V 
-® 






,1 1 


J 1 


J 












































EPROM 



EPROM 



EPROM 

EPROM 







1 



2 



3 






































































































































OE 



OE 



5e 

22 


OE 








?4 


24 


24 



25 


25 


25 



26 


26 


26 



27 


27 


27 



28 


28 


28 










FF1.FF2- ICI = 74LS74 


Figure 1. The circuit of 
the EPROM selector is 
quite uncluttered: with 
the exception of a few 
lines, all identical EPROM 
pins are interconnected. 


sockets); when 27XX EPROMs are used, A 
must be connected to B — when 25XX 
EPROMs are used, A must be connected 
to O; 

■ if 24-pin EPROMs are used, bridge 
VCC24 must be wired in — for 28-pin 
EPROMs, bridge VCC28; in the former 
case, C2 may be omitted — in the latter, 
C3. 

The selection logic consists of two 
bistables, FF1 and FF2, and dual 2-line 
binary decoder IC2. Its operation is made 
clear in figure 2. During time T1 the com- 
puter writes data into the RAM range: 
writing pulses NWDS (negative write data 
strobe) do not affect the selection logic. 
Note that these pulses in some computers 
may be symbolized by R/W, WR, or 
others: you’ll find this in your operating 
instructions. 

During time T2 the computer will be 
active on the, now multiplied, EPROM. 
Decoder 2 is then cleared via decoder 1: 


2 


I \yj~\j — r 

— \j 

J L 



K-TLT I 



„.. ,J W W ; 

T1 T2 J. T3 


85007-2 


one of the outputs 2Y 0. . . 2Y 3 becomes 
logic low and this causes one of the 
EPROMs to be selected — which one 
depends on the output state of the 
bistables. 


Figure 2. This timing 
diagram clarifies the 
operation of the selection 
logic. 


elektor mdia march 1985 3.47 







EPROM selector 


3 


Parts list 

C1,C2 or C3 = 100 n* 

IC1 = 74LS74 
IC2 = 74LS139 
PCB 85007 

5 sockets, 24-pin or 28-pin 
as required — see text 
2 DIL plugs, 24-way or 
28-way, as required, with 
spring-loaded contacts 
ribbon cable, 24-way or 
28- way, as required. 

* = total 2, see text 



Figure 3. The printed cir- 
cuit board of the EPROM 
selector. 


The inputs of the bistables are connected 
with data bus lines D0 and Dl. During 
time T3, the computer is instructed to 
write into the EPROM range — this may, 
for instance, be through _a POKE com- 
mand. The NWDS and OE lines first 
become logic 0 and then go high again. 
This actuates output 1Y0 which also first 
goes logic low and then becomes 1 again. 
As this output is connected to the clock 
inputs (CLK) of the bistables, the infor- 
mation on the appropriate data bus line is 
passed on to the relevant bistable at the 
leading edge of the pulse on 1Y0 Table 1 
gives the relation between the byte at the 
data bus (hexadecimal and decimal), the 
logic level at data bus lines D0 and Dl, 
and the EPROM next in line to be 
enabled. The bistables ensure that the 
selected EPROM remains active until a 
fresh word is written into the EPROM 
address range. 

Apart from the wire bridges already men- 
tioned, six more are required as shown on 
the printed circuit in figure 3. Nothing 
further needs to be said about the con- 
struction other than that the supply 
voltage is derived from the computer via 
the ribbon cable. 

It will be clear from the nature of the cir- 
cuit that if, for instance, the original 


EPROM is a type 2732, the additional 
EPROMs must be of the same type. 

Typical applications 

■ Loading of an extensive operating 
system from the EPROMs instead of 

floppy disk into RAM. This can be done 
very rapidly after which the operating 
system can no longer be lost accidentally. 
You need to write a relevant program 
suited to your computer, of course, and 
this presumes a certain familiarity with 
programming. 

■ Four banks of utility programs instead 
of one, or up to four programming 

languages may be permanently loaded 
onto the main store of the computer. 

■ change-of-character sets on a VDU card 
or in the character generator of a 

single-board computer. 

■ Change-over between various keyboard 
layouts (change-over by pressing a 

push-button which connects OE to earth 
and the simultaneous operating of a 
character key. The negative strobe pulse 
is connected to NWDs, and D0 and Dl to 
the relevant data lines). 

■ Various games may be accessed by a 

short instruction instead of having to load 
them from a cassette. M 


3.48 elektor india march 1985 


The principle of ‘Easy music’ is ex- 
tremely simple, as can be seen from the 
block diagram of figure 1. The ‘mu- 
sician’s’ whistle is picked up by a crystal 
microphone and amplified by op amp 
A 1 . A portion of the signal is fed to an 
envelope follower, which rectifies and 
filters it to produce a positive voltage 
that follows the amplitude envelope of 
the input signal. The signal is also fed to 
two limiting amplifiers, which convert 
the variable amplitude sinewave of the 
input signal into a constant amplitude 
squarewave having the same frequency 
as the input signal. This squarewave is 
used to clock a binary counter whose 
division ratio can be set to 2, 4, 8 etc., 
so that the output is one, two, three etc. 
octaves below the input signal. 

The counter output is used to switch 
transistor T1 on and off, and the col- 
lector signal of T1 is fed to the output 
amplifier A4. Since the collector 
resistor of T1 receives its supply from 
the output of the envelope follower, the 
amplitude of the collector signal, and 
hence of the output signal, varies in 
sympathy with the amplitude of the 
original input signal. 

The output is therefore a squarewave 
whose frequency may be one or more 
octaves lower than the input signal and 


easy music 


whose amplitude dynamics follow the 
amplitude of the input signal. 

Complete circuit 

The complete circuit is given in figure 2 
and the sections of the circuit shown in 
the block diagram are easily identified. 
The output of the crystal microphone is 
fed to PI, which functions as a sensitivity 
control. A1 is connected as a linear 
amplifier with a gain of approxi- 
mately 56. A portion of the output 
signal from A1 is rectified by D1 and 
the resulting peak positive voltage is 
stored on C4. The output signal from 
A1 is further amplified by A2 and A3, 
the combined gain of A1 to A3 being 
sufficient to cause limiting at the 
output of A3, even with very small 
input signals. P2 is used to adjust the 
gain of the limiting amplifier so that 
limiting just occurs with the smallest 
input signal, this avoiding limiting 
caused by extraneous noises. 

The output of A3 is used to clock a 
CMOS binary counter, whose division 


For those who do not have the 
time (or perhaps the patience) to 
master a musical instrument, but 
would nonetheless like to make 
their own music, this simple 
circuit may provide the answer. 
The only musical accomplishment 
necessary is the ability to whistle 
in tune. 

P.J. Tyrrell 



ratio may be set by means of SI. The 
output of IC2 switches transistor T1 on 
and off. Since the collector resistor of 
T1 (R6) receives its supply voltage from 
C4, the amplitude of the collector signal 
varies in sympathy with the input signal. 
This signal is amplified by a small audio 
power amplifier built around A4, which 
drives a small loudspeaker. 

Additions to the basic circuit 

However, the possibilities do not end 
there. The more ambitious constructor 
may wish to add filters and other circuits 
to produce different output waveforms 
which will extend the tonal possibilities 
of the instrument. Such variations on 
the basic design are, however, beyond 
the scope of this short article, and are 
left to the ingenuity of the individual 
reader. M 



elektor india march 1985 3.49 







RLC meter 


to quickly 

identify 

unknown 

resistors, 

inductors and 

capacitors 


If we were to make a list of equipment for an electronics laboratory 
this RLC meter would feature high in the order of preference. Possibly 
it would be second only to the multimeter. In a way this is a sort of 
multimeter: a simple instrument that can measure the values of 
resistors, inductors and capacitors. The meter is reasonably accurate, 
easy to build and even quite inexpensive. In short, it is simply too 
good an opportunity to be missed. 


At times Elektor has been criticized for 
the lack of test instruments published in 
the magazine. That criticism is no longer 
justified as we have published (among 
other projects) a capacitance meter, pulse 
generator, function generator and fre- 
quency meter all in the last year. Con- 


Table 1. 


measuring ranges 



R 

L 

C 

1 

1. . .10 Q 

0.1. . .1 mH 

1. . .10 pF 

2 

10. . 100 Q 

1...10 m h 

10 . 100 pF 

3 

100Q...1 k Q 

10. . 100 mH 

100 pF. . .1 nF 

4 

1...10 kQ 

100 11H...I mH 

1. . .10 nF 

5 

10. . .100 kQ 

1. .10 mH 

10 100 nF 

6 

100 kQ. . .1 MQ 

10. . .100 mH 

100 nF. . .1 pF 

7 


100 mH. . .1 H 

1 10 pF 


sidering the scope and quality of these 
designs we consider that no mean 
achievement. 

This RLC meter is a worthy addition to the 
series although it is somewhat different to 
the rest. This is the inevitable result of its 
multimeter-like character. It could be 
made more specialized, like the rest of 
the test instrument series, but that would 
only serve to taint the simple form of the 
meter. 

If you want to measure capacitors with a 
large degree of accuracy a good 
capacitance meter is the logical choice: 
for coils a self-inductance meter is called 
for and for resistors a resistance meter is 
in order. Should you not have the money 
for three test instruments or have no need 
for absolute accuracy then this RLC meter 
is the instrument for you. 

The layout 

Every RLC meter follows more or less the 
same pattern, the differences arise in the 


3.50 elektor India march 1985 


way in which that design is implemented. 
The build-up of our RLC meter is shown 
in figure 1 and clearly the layout is very 
simple. 

An oscillator feeds a specific signal to an 
impedance bridge. One branch of the 
bridge consists of the resistor, inductor or 
capacitor (Zx) that is to be measured and 
a reference impedance (Zref). The other 
side is made up of a fixed resistor (R) and 
a potentiometer (P). The voltages at the 
junctions of each branch are detected and 
fed to a comparator that drives two LEDs. 
If the voltages at the junctions are differ- 
ent only one of the LEDs will light. When 
the bridge is balanced by means of the 
potentiometer both LEDs light. The value 
of the resistor, coil or capacitor under test 
can then be determined from the value of 
Zref (known) and the position of P. 

The only thing that is then needed is a 
number of accurate switchable reference 
resistors, inductors and capacitors for Zref 
and a suitable scale for the potentiometer. 
This brings us to. . . 

The circuit diagram 

The layout from the block diagram is eas- 
ily recognised in the circuit diagram of 
figure 2. We will deal with each of the 
sections separately, saving the actual 
bridge until last as this requires the most 
detailed comment. 

The detectors are found at the bottom of 
the diagram. These are IC1/D1 and 
IC2/D2 and the components associated 
with each. The inputs to the detectors (the 
non-inverting inputs of the op-amps) are 
connected to the junctions of R11/R12 and 
S4/Rx- A close look will show that these 
are also the junctions of each branch of 
the bridge. 

The output signals from the detectors are 
fed to op-amp IC3, which serves as a 
comparator. This comparator drives indi- 
cator LEDs D3 and D4 via transistors T4 
and T5. 

The power supply section is seen at the 
top right-hand corner of the diagram. This 
has the usual layout and requires no 
further comment. Just left of the power 
supply is the oscillator. This is based on 
Tl, T2 and T3 and is a bit more com- 
plicated than might seem strictly 
necessary. The reason is that the oscillator 
must supply quite a lot of power in order 
to be able to cope with the low 
impedance of the loads in some of the 
ranges. For the same reason a (star- 
shaped) heatsink must be fitted to power 
transistor T3. The frequency of oscillation 
is about 18 kHz, A higher frequency 
would have been useful in measuring 
small values of inductance and 
capacitance but would prove an unaccept- 
able load for the oscillator when measur- 
ing large capacitors. By the same token a 
lower frequency would be advantageous 
for measuring large inductors and 
capacitors but the oscillator would be ^s 
good as short circuited when measuring 
small inductors. The frequency of 18 kHz 


1 



provides a reasonable compromise. 

Now all that is left is the middle section of 
the diagram: the actual bridge network. 
The ‘fixed’ branch of the bridge is seen at 
the left-hand side. The resistor from the 
block diagram, R, is formed by RIO and 
Rll in series, and potentiometer P is made 
up of R12 and PI. 

In the other branch of the bridge we see 
two connection points for the resistor, 
inductor or capacitor that is to be tested 
(Rx, Lx, Cx — Zx in figure 1). The refer- 
ence impedance, Zref. is almost a 
separate section all of its own. We want to 
be able to measure resistors, coils and 
capacitors so we need a number of refer- 
ence examples of each type of compo- 
nent. The number of each type needed 
depends on the number of ranges 
desired. We went the whole hog with this 
design by giving it seven ranges and used 
the most accurate components we could 
find. The meter will work if the reference 
components have a high tolerance but it 
will not be as accurate. The type of com- 
ponent to be measured, R, L or C, is 
chosen with S4. The desired range can 
then be selected using SI, S2 or S3. The 
measuring range given in each case is 
indicated in table 1. 

While we are on the subject of measuring 
ranges there is one point we should make. 
Three of the reference components, L7, 

Cl and R7 are marked with an asterisk in 
figure 2, and with good reason. The 


RLC meter 


Figure 1. This block 
diagram shows the main 
parts of the RLC meter: 
an oscillator, a bridge 
network, two detectors 
and a comparator. The 
LEDs indicate when the 
bridge is balanced. 


elektor mdia march 1985 3.51 






Figure 2. The circuit 
diagram. 


2 


Parts list 

Resistors: 

R1 = 10 S, 1% 

R2, RIO, R12 = 100 Q, 1% 
R3, R11 = 1 k, 1% 

R4 = 10 k, 1% 

R5 = 100 k, 1% 

R6 = 1 M, 1% 

R8, R18, R20 = 1 k 
R9 = 10 a 
R13 = 10 M 

R14, R15, R17, R19 = 100 k 
R16 = 220 a 
R21 = 1k2 

PI = 1 k lin. pot, 5% or 
wirewound 
P2 100 k preset 

Capacitors: 

Cl « • 

C2 - 100 p, 5% (or 1%) 

C3 = 1 n, 5% (or 1%) 

C4 = 10 n, 5% (or 1%) 

C5 - 100 n, 5% (or 1%) 

C6 - 1 p. 5% (or 1%) 

C7a, C7b = 22 p/25 V, 5% 
lor 1%l 
C8 = 270 n 

C9, C17, C18, C19 = 100 n 

CIO = 470 p 

C11 = 330 p 

C12 = 470 n 

C13, C14 = 47 n 

C15, C16 = 1000 p/16 V 

Inductors: 

LI = 1 pH 
L2 10 pH 
L3 = 100 pH 
L4 = 1 mH 
L5 = 10 mH 
L6 = 100 mH 
L7 = 1 H - 

L8a,b, =50+10 turns of 
0.25 mm ISWG 33) CuL on 
a pot core of 18 x 11 mm 
(Al = 2501 

or 27 + 5 turns of 0.25 mm 
CuL on a twin-hole ferrite 
core of 14 x 14 x 8 mm 
(with 3.5 mm diameter 
holes) 

Semiconductors: 

D1, D2, D9. D10 = 1N4148 
D3 = LED, green 
D4, Dll - LED, red 
D5 D8 = 1 N4001 
T1 = BF256B, BF245B 
T2 = BC557B, BC559C 
T3 - BC140 16, BC141 16. 
2N2219 
T4 = BC557B 
T5 - BC547B 
IC1, IC2 = CA3140 
IC3 = CA3130 

Miscellaneous: 

FI = fuse, 50 mA 
SI . . . S4 - single-pole 
12-way rotary wafer switch 
S5 - double-pole mains 
switch 

Trl = mains transformer, 

2 x 6 V/100 mA 
1 off T039 type heatsink for 
T3 

4 off switch knobs with 
marker line (SI. . .S4) 

1 off switch knob with 
marker needle (PI) 

1 off case, dimensions 
(minimum) 190 (W) x 104 
(D internal) x 62 (H rear) 
to 33 I H front) 

* - see text 



largest value inductor, L7, may not be 
available as a close-tolerance item but that 
is not necessarily a problem. Using a 
higher-tolerance part will simply make this 
range less accurate. 

The difficulty with Cl and R7 is quite dif- 
ferent. In both of these ranges the 
capacitance and resistance of the tracks 
on the printed circuit board have quite a 
large effect. The problem can be solved 
for Cl by using a trimmer here and 
adjusting this to give exactly the value of 
capacitance needed between the com- 
mon pole of S2 and contact number 2 of 
S4. (We will return to this point at the end 
of this article.) The largest value resistor, 
R7, is not available at 1% with a value of 
10 MQ so it will have to be omitted from 


the parts list, leaving us with an upper 
measuring range extending up to 1 MQ. 


Construction 

With the exception of the parts already 
mentioned the components for the RLC 
meter should not be a problem. One of 
the inductors, L8, will have to be wound. 
Details are given in the parts list. 

The printed circuit board used for this 
RLC meter is shown in figure 3. All com- 
ponents except the mains transformer and 
power switch can be fitted directly onto 
the board. The photograph in figure 4 
shows what the finished board looks like. 
The meter can be mounted in any sort of 


3.52 elektor india march 1985 






case you like but it is only logical to fit 
the printed circuit board directly behind 
(or under) the front panel. Five points on 
the copper side of the board in the 
middle of each of SI. . .S4 and PI indicate 
the centre of the switches and poten- 
tiometer. In this way the board can be 


considered as a template for the front 
panel. Provision must be made for fitting 
the LEDs, power switch and input sockets, 
of course, but their positions are not really 
critical. To fit into the case we used for 
the meter the comers of the printed cir- 
cuit board had to be filed off. The mains 


RLC meter 


Figure 3. Almost all the 
components for the RLC 
meter are mounted on 
this printed circuit board. 
The board is also used as 
a template for drilling 
holes in the front panel. 


elektor mdia march 1985 3.53 






RLC meter 


transformer was fixed to the back panel. 
There are a few other small, but important, 
points regarding construction: 

■ Some of the components, such as L6, L7 
and L8, may appear too high to fit 

between printed circuit board and front 
panel. If this is the case the components 
in question can be mounted on the 
reverse side of the board. 

■ There are two methods of connecting 
the rotary switches and the poten- 
tiometer to the board. The soldering lugs 
can be soldered directly to the board and 
this will also help to keep it solidly in pos- 
ition. Alternatively, mount the switches 
and pot onto the front panel and use wires 
to make the necessary connections. 

■ Keep all the wiring, especially from the 
input sockets, as short as possible. If 

the input sockets tend to foul the board a 
couple of holes can be drilled in the 
board through which the sockets can 
protrude. 

■ The mains switch mounts directly onto 
the front panel. Above LED Dll on the 

printed circuit board there is a small hole. 
The mains wires to the switch pass 
through this hole. 

■ There is also a hole drilled under P2 on 
the board. This is used to trim the 

preset after the board has been 
assembled. 


Using the meter 

Before the meter can be used the rotary 
switches and the potentiometer must be 
provided with suitable scales. For this we 
refer you to figure 5, which shows one 
possible front panel layout. A double 
scale is needed for PI as the graduations 
for capacitors run in the opposite direc- 
tion to those for resistors or coils. The 
graduations for PI are linear for almost all 
ranges, only deviating in some of the 
upper ranges. We will return to this point 
in the section on calibration. 

Using the RLC meter is very easy: 

■ Connect the component to be 
measured to the input sockets, keeping 

the wires or leads as short as possible. 

■ It is reasonable to assume that the type 
of component to be tested is known so 

the appropriate position of S4 (R, L or C) 
can be chosen. 

■ Generally you will have some idea of 
the value of the component so the 

appropriate range is selected with the 
relevant switch, SI, S2 or S3. 

■ Potentiometer PI is then turned until 
both LEDs (D3 and D4) light. 

■ If that does not happen the range is 
incorrect so other ranges will have to 

be tried until the one is found in which 
the two LEDs do light when PI is turned. 

■ With both LEDs lit simply read off the 
value that PI is pointing to and multiply 


Figure 4. This photograph 
shows clearly how all the 
various components fit 
onto the printed circuit 
board. 



3.54 elektor india march 1985 


5 


RLC meter 



it by the range selected. The result is the 
value of the component that was 
measured. 


Calibration 

Calibrating the circuit is a matter of 
adjusting out the offset of IC3, which is 
very simple. Short -pins 2 and 3 of 1C3 
together and trim preset P2 until LEDs D3 
and D4 are both off. 

Before starting on the actual scale cali- 
bration procedure there is something we 
would like to point out. If the most 
accurate of reference components 
(Rl. . ,R6, LI. . .L7, Cl. . .C7) are used an 
accuracy of 1% can be achieved. Cali- 
bration must then also be carried out with 
1% components. If standard components 
(5% tolerance) are used for calibration the 
meter will be less accurate but this should 
still be sufficient for most purposes. 

In the 'normal' ranges (it will soon 
become clear which they are) there is no 
real need for calibration and the scale 
indicated in figure 5 can be used. To 
verify this scale for each range a compo- 
nent whose value is known can be con- 
nected to the input sockets and when PI 
is adjusted so that both test LEDs are lit 
the pointer should indicate the right value. 
Three ranges could be considered as 
‘problematic’, namely range 6 resistors 
(100 k . . . 1 M), range 1 capacitors (1 ... 10 p) 


and range 1 inductors (0.1 ... 1 >jH). If these 
ranges are not to be used then there is no 
problem. If they are wanted, however, a 
separate scale will have to be made for 
each range as the graduations are no 
longer linear. In resistor range 6 an 
‘infinite’ resistance is indicated not at the 
end of the scale but rather at about 3 A of 
full scale. The same applies for the ‘0’ in 
capacitor range 1, while the ‘O' in inductor 
range 1 corresponds to about 'A way from 
the start rather than the expected position. 
A large number of components within 
these ranges will be needed to work out 
the graduations for Pi’s scale. Place each 
value of component in the test (input) 
sockets in turn and mark the correspond- 
ing position on the scale. In this way the 
three scales can be made. 

We have already mentioned that there 
could be a problem with Cl. Stray or 
parasitic capacitance caused by the actual 
printed circuit board tracks can cause the 
value to deviate from the anticipated 10 p. 
The way around this problem is to use a 
fixed capacitor of 6.8 p with a 3 p trimmer 
in parallel with it. Connect an accurate 
10 p calibration capacitor to the input 
sockets and adjust the trimmer until the 
10 p corresponds exactly to the start (left- 
most position) of the scale of PI. N 


Figure 5. Here we see one 
possible layout for the 
meter's front panel. The 
text of the article pro- 
vides some information 
about the scale used for 
potentiometer PI. 


elektor india march 1985 3.55 




programmable 
keyboard encoder 


The large scale integration (LSI) alphanumeric keyboard encoder ICs 
are now quite well known, so much so, in fact, that they have 
become almost classical. Their good features have been loudly 
heralded but they also have a number of unfavourable characteristics. 
The keyboard matrix is scanned at a high frequency and this causes 
some 'pollution' in the form of HF radiation; the matrix configuration 
coded into ROM is fixed rigidly; there is much duplication, with the 
result that there are far fewer usable codes than there are positions 
in the matrix. These disadvantages are totally unknown to our static 
encoder, whose versatility enables it to be used for a wide range of 
applications. It permits the standard alphanumeric keyboard 
arrangement (QWERTY) to be implemented, of course, but it also 
makes it possible to use any specialised user-defined layout. 


programmable 
keyboard encoder 


a static 80 -key 
matrix that can 
be used for any 
application 


from an idea 
by C. Bajeux 


A discrete programmable encoder could 
provide an alternative for the encoder ICs 
that are commonly used in alphanumeric 
keyboards. An EPROM is used in the sec- 
tion that generates the output codes so 
every imaginable configuration is both 
feasible and easily realised. There is no 
duplication in the matrix except that a key 
may be used more than once. In general 
this only applies to the keys for SHIFT, 
CTRL, and numbers 0 ... 9 and letters 
A . . . F, which are often found on a hexa- 
decimal keyboard as well as the main one. 
The effect of the SHIFT and CTRL keys on 
the hexadecimal keypad, which could 
cause problems, can easily be neutralised. 


Simultaneous high and low logic 
levels 

One of the most striking aspects of the 
circuit diagram of figure 1 is the presence 
of CMOS ICs and an auxiliary voltage of 
18 V among the TTL circuits. This mixture 
allows the logic level on the matrix 
columns to be different from that on the 
rows, although both are based on the 
same voltage. 

There is no keyboard scanning in the nor- 
mal sense of the expression. The 80-key 
matrix is located between two priority 
encoders, one being a CMOS 8-bit device 
(ICS), the other a 10-bit TTL chip (IC6). 
Strobe pulses (STROBE and STROBE) are 
generated by gates Nl, N2, N3 and NS, 
which activate IC3 so that it stores the 
data output by the EPROM. There is a 
special facility in the circuit, provided by 
gates N4, N6, N7 and N8, to enable the 
keyboard to be addressed directly on the 
computer's data bus without first having to 
pass through a peripheral IC such as a 
VIA or PIO. 

All the columns in the matrix are forced 


low by means of resistors R1 . . . R8. When 
a key is pressed one of the columns goes 
high and the binary code corresponding 
to one of lines X0. . .X7 then appears on 
the A0. . . A2 outputs of encoder ICS. 

Row lines Y0. . ,Y9, on the other hand, are 
forced high by resistors R9. . .R18 in the 
rest state. As soon as a key is pressed the 
relevant line goes low. The appropriate 
binary code then appears (inverted) at the 
output of 10-bit priority encoder IC6. 

It may seem a bit strange to note that in 
this circuit the same voltage is a logic low 
level for one line and a high level for 
another line. This arises from the fact that 
when a key is pressed the voltage at the 
appropriate column/row intersection is 
about 4 V. This is a ‘high’ for IC5, which 
has a 5 V supply, but a ‘low’ for IC7 (or 
IC8), whose logic levels are determined 
with respect to the 18 V on pin 16. The 
output logic levels for IC7 and IC8 are 
fixed with respect to the voltage at pin 1, 
which is 5 V here in order to ensure com- 
patibility with the TTL ICs. Note in pass- 
ing that input 0 of IC6 is not used 
although this chip does encode 10 lines. 
The tenth matrix line (Y0) is not connected 
to the 74LS147. When none of the other 
nine lines is active the output of IC6 is 
‘1111* (the inverse of ‘0000’), which cor- 
responds to the binary code for line Y0. 
You may wonder what would happen to 
these voltages if several keys were 
pressed at the same time, particularly if 
they are in the same column. The greater 
the number of 33 k resistors in parallel on 
the same column the higher the voltage 
input to IC5. For this reason each of the 
columns is fitted with a protection diode 
(Dl. . ,D8) to limit the voltage to 5.6 V. Con- 
sequently the danger of destroying IC5 is 
removed. Furthermore the code output 
from the encoders if several keys are 
pressed at the same time is always that for 


3.56 elekior mdia march 1985 



1 


programmable 
keyboard encoder 



the key at the highest co-ordinates of the 
X/Y matrix. 


The code conversions 

The ASCII codes corresponding to every 
position in the matrix are stored in a 2716 
EPROM (IC4). As could be expected, the 
two priority encoders provide a binary 
code that is used to address the EPROM. 
There are four codes corresponding to 


each key: the key itself, the key with 
SHIFT pressed, the key with CTRL and 
the key with both SHIFT and CTRL 
pressed. The latter two keys are con- 
nected to A3 and A4 respectively, and as 
they can be pressed individually or simul- 
taneously this is an easy way of increasing 
the number of codes that can be 
accessed. We will see shortly how the 
contents of the EPROM is,arranged but 
first we must have a look at the upper part 
of figure 1. 


Figure 1. This program- 
mable keyboard encoder 
can be accessed directly 
from a microprocessor’s 
data bus. If this facility is 
not used the wire bridge 
linking pin 1 of N4 and 
pin 3 of N6 must be 
removed and replaced by 
a link connecting both 
inputs of N4 to the out- 
put of N5. 


elektor mdia march 1985 3.57 





programmable 
keyboard encoder 


2 


Figure 2. This diagram 
shows how the keys in a 
normal 'alphanumeric 
plus hexpad' keyboard are 
arranged in the matrix 
rows. Other layouts are. 
of course, possible. 



Table 1. 




X0 

XI 

X2 

X3 

X4 

X5 

X6 

X7 | 

X0 

XI 

X2 

X3 

X4 

X5 

X6 

X7 

col. 


function 


0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

A i 

B 

C 

D 

E 

F 

line 


c + s 

0C 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

0D 

1C 

0C 

30 

31 

32 

33 

2E 

Y9 

C 

s 

0D 

0D 

1C 

01 

30 

31 

32 

33 

2E 

0D 

1C 

0C 

30 

31 

32 

33 

2E 

N 

c + s 

0E 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

38 

39 

41 

42 

34 

35 

36 

37 

Y8 

C 

s 

0F 

38 

39 

41 

42 

34 

35 

36 

37 

38 

39 

41 

42 

34 

35 

36 

37 

N 

c + s 

10 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

43 

44 

45 

46 

Y7 

C 

s 

11 

FF 

FF 

FF 

FF 

43 

44 

45 

46 

FF 

FF 

FF 

FF 

43 

44 

45 

46 

N 

c + s 

12 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

0B 

0A 

20 

00 

00 

IF 

08 

09 

Y6 

C 

s 

13 

08 

0A 

20 

3E 

3F 

5F 

08 

09 

0B 

0A 

20 

2E 

2F 

5F 

08 

09 

N 

c + s 

14 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

1 A 

18 

03 

16 

02 

0E 

0D 

00 

Y5 

C 

s 

15 

5A 

58 

43 

56 

42 

4E 

40 

3C 

7A 

78 

63 

76 

62 

6E 

6D 

2C 

N 

c + s 

16 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

0F 

10 

00 

IB 

0C 

00 

00 

ID 

Y4 

C 

s 

17 

4F 

50 

60 

7B 

4C 

28 

2A 

50 

6F 

70 

40 

58 

6C 

38 

3A 

70 

N 

c + s 

18 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

14 

19 

15 

09 

07 

08 

0A 

0B 

Y3 

C 

s 

19 

54 

59 

55 

49 

47 

48 

4A 

48 

74 

79 

75 

69 

67 

68 

6A 

6B 

N 

c + s 

1 A 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

11 

17 

05 

12 

01 

13 

04 

06 

Y2 

C 

s 

18 

51 

57 

45 

52 

41 

53 

44 

46 

71 

77 

65 

72 

61 

73 

64 

66 

N 

c + s 

1C 

, FF 

i FF 

FF 

FF 

FF 

FF 

FF 

FF 

00 

00 

00 

00 

IE 

1C 

7F 

0A 

Y1 

C 

s 

10 

28 

29 

00 

30 

7E 

7C 

7F 

0A 

38 

39 

30 

20 

5E 

5C 

7F 

0A 

N 

c + s 

IE 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

FF 

IB 

00 

00 

00 

00 

00 

00 

00 

Y0 

C 

s 

IF 

IB 

21 

22 

23 

24 

25 

26 

27 

IB 

31 

32 

33 

34 

35 

36 

37 

N 


Table 1. Using an EPROM 
to aid addressing the 
matrix positions ensures 
that there is no dupli- 
cation, no 'holes' and no 
incongruities. Four differ- 
ent codes can be 
attributed to each of the 
keyboard's 80 keys. 


The data output from IC4 is latched into 
IC3, whose outputs can be connected 
directly to a data bus. (When the 74LS374 
is not enabled its Ql. . Q8 outputs have a 
high impedance.) This latching is essential 
as the data must remain stable when the 
key is released. Information is input to IC3 
from its Dl. . ,D8 lines when the CLK input 
detects a falling edge — provided in this 
case by IC5’s enable output (EO) via 
debounce network Nl. . .N3. As long as 
no key is pressed pin IS of IC5 is high 
and pin 14 (GS) is low. Pressing a key 
causes these signals to invert (but with 
overshoot!); they return to the quiescent 
state as soon as the key is released. 

The EO and GS (group select) outputs are 
used as the basis for the STROBE and 
STROBE pulses and also for the pulse 
used to clock eight-bit latch IC3. The 
STROBE signal is also fed via N4 to enable 
the addressing of IC3. 

The keyboard can, as we have already 
said, be accessed directly by a micropro- 
cessor’s data bus. This is only possible 
during the strobe pulse, which allows N4 
to pass on the addressing signal provided 
by N6. The output of this latter gate can 


only be high if both the read signal (RT5) 
and the address decoding signal (ADR) 
are active (‘0’ in each case). 

The keyboard can be programmed for 
polling mode, in which the processor 
itself examines the state of the STROBE 
line, or interrupt mode, whereby flip-flop 
N7/N8 supplies the interrupt signal (INT 
or INT) when a key is pressed. If capaci- 
tor CS is replaced by a wire bridge flip- 
flop N7/N8 is only initialised when the 
processor addresses the keyboard. In fact 
the flip-flop is initialised only when the 
key is released so this makes it easy to 
implement a repetition function controlled 
by the software. If, on the other hand, C5 
is included in the circuit the flip- flop i s 
initialised as soon as the HD and ADR 
signals become active. 

Programming the EPROM 

Each key has four corresponding 
addresses in the EPROM: first is the key 
together with both SHIFT and CTRL, then 
the key with CTRL only, followed by the 
key with SHIFT only and finally the single 
key on its own. It is also conceivable to 


3.58 elektor mdia march 1985 







Table 2. 


programmable 
keyboard encoder 


C = CONTROL (SI); S = SHIFT (S2); N = NORMAL 


X0 

~~xT~ 

X2 

X3 

X4 

X5 

X6 

x7 

CR 

FS 

FF 

0 

1 

2 

3 

■ i c 

Y9 CR 

FS 

FF 

0 

1 

2 

3 

S 

CR 

FS 

FF 

0 

1 

2 

3 

N 

8 

9 

A 

B 

4 

5 

6 

7 C 

Y8 8 

9 

A 

B 

4 

5 

6 

7 S 

8 

9 

A 

B 

4 

5 

6 

7 N 

lit . 

F2 

F3 

F4 

C 

D 

E 

F C 1 

Y7 




C 

D 

E 

F S 





C 

D 

E 

F N 

VT 

LF 

SP 

NUL 

NUL 

US 

BS 

HT 1 C 

Y6 VT 

LF 

SP 

> 

? 

— 

BS 

HT S 

VT 

LF 

SP 

• 

/ 


BS 

HT N 

SUB 

CAN 

ETC 

SYN 

STX 

SO 

CR 

NUL C 

Y5 Z 

X 

C 

V 

B 

N 

M 

< s 

2 

X 

c 

V 

b 

n 

m 

• N 

SI 

DLE 

NUL 

ESC 

FF 

NUL 

NUL 

GS C 

Y4 0 

P 


{ 

L 

+ 

* 

> S 

o 

P 

@ 

1 

1 

; 


1_ | N 

DC4 

EM 

NAK 

HT 

BEL 

BS 

LF 

VT C 

Y3 T 

Y 

U 

1 

G 

H 

J 

K S 

t 

y 

u 

i 

g 

h 

j 

k N 

DC1 

ETB 

ENQ 

DC2 

SOH 

DC3 

EOT 

ACK C 

Y2 Q 

W 

E 

R 

A 

S 

D 

F ' S 

q 

w 

e 

r 

a 

s 

d 

f N 

NUL 

NUL 

NUL 

NUL 

RS 

FS 

DEL 

LF 1 C 

Y1 ( 

) 

NUL 

= 


1 

DEL 

LF S 

8 

9 

0 

- 

A 

\ 

DEL 

LF N 

ESC 

NUL 

NUL 

NUL 

NUL 

NUL 

NUL 

NUL C 

Y0 ESC 

! 

" 

# 

$ 

% 

£f 

S 

ESC 

1 

2 

3 

4 

5 

6 

7 N 

Y0 








Y9 







■ ■■ Is + c| 


have several blocks of different codes 
whereby the block selected depends on 
the logic levels applied to lines A9 and 
A10 of the EPROM. In our circuit diagram 
this possibility is not used so these two 
address lines are kept low. When pro- 
gramming the EPROM it is essential to 
bear in mind that the outputs of IC6 are 
inverted. The lowest accessible address 
(0C0HEX) corresponds to key X0-Y9 when 
SI and S2 are closed. The highest address 
is 1FFHEX, which corresponds to key 
X7-Y0 with SI and S2 both open. Starting 
with the lowest address the first codes 
programmed correspond to row Y9 (mov- 
ing from left to right) with SI and S2 
closed. These are followed by the same 
keys but this time with SI closed and S2 
open, then the same again except that SI 
is open and S2 is closed, and finally the 
same row with both SI and S2 open. The 
second row, Y8, starts at address 0E0HEX 
with the leftmost key, SI and S2 being 
closed. The same sequence is then fol- 
lowed as for row Y9. 

This procedure was used to formulate 
table 1 for an alphanumeric keyh oard 
such as that shown in figure 2. 1 he layout 


is in no way unusual as it is only intended 
as an example. Note that the keys of the 
hexadecimal keypad are not effected by 
the positions of SHIFT or CRTL. The lower 
part of this table is open as we have only 
dealt with the ‘normal’ use of the keys. If 
an application requires extra codes to be 
generated this can easily be done by pro- 
gramming an additional code for each of 
the 80 keys. The appropriate code will 
then be output when any key is pressed 
by the same time as both SHIFT and 
CRTL. These codes will then be 
substituted for the FFs in table 1 at 
adresses 0C0. . 0C7, 0E0. . .0E7, 100. . .107, 
120. . . 127 and so on up to 1E0. . . 1E7. K 


Table 2. It is clear from 
table that the CTRL and 
SHIFT keys do not effect 
the hexadecimal keys on 
the separate keypad. No 
codes have been included 
in this table to cater for 
special functions gener- 
ated by pressing a key at 
the same time as both 
SHIFT and CTRL. 


elektor mdia march 1985 3.59 



k M A A 


digital graphic equalizer 

Among the many new ICs from 
National Semiconductor is one that 
combines microprocessor and audio 
techniques. This is the LMC 835: a 
monolithic digitally controlled graphic 
equalizer 1C, which is manufactured 
in LSI (large-scale integration) CMOS 
technology and is intended for use in 
high performance audio applications. 
Basically, the LMC 835 consists of a 
logic section and a signal-path sec- 
tion made up of analogue switches 
and thin-film silicon-chromium 
resistor networks. Used with external 
resonator circuits, the 1C makes a 
stereo equalizer with seven bands, 
each with a ± 12 dB or a ± 6 dB 
gain range in twenty-four steps. A 
block diagram of the interior of the 
LMC 835 is shown in figure 1. 

The control function is carried out by 
three digital input signals: the clock, 
a strobe, and a serial data control 
word. The control data is divided 
into the band selection data, referred 
to as DATA I, and the gain selection 
data, DATA II. These data sets may 
be provided by a microprocessor and 
are entered in serial format in con- 
junction with the strobe as illustrated 
by the waveform timing diagram in 
figure 2. 

The truth tables for the data sets are 
shown in figure 3. It will be seen 
that bit D7 of the data word deter- 
mines a band selection or a gain 
selection; it is high for DATA I and 
low for DATA II. Bit D6 is used only 
during the gain selection (DATA II) to 
effect either a boost or a cut in the 
gain response. Bits D4 and D5 in the 
DATA I band selection table deter- 
mine the gain selection response 
characteristics. 

The audio signal path of the 
LMC 835 is designed for very low 
noise and distortion to result in very 
high performance compatible with 
PCM (pulse code modulation) audio 
applications. As well as a graphic 
equalizer, the LMC 835 can be used 
for many other applications, includ- 
ing volume control with very low 
total harmonic distortion, a mixer, 
tape equalization, and special-effect 
j circuits for musical instruments. 

The circuit diagram in figure 4 shows 
a seven-band stereo equalizer. It 
includes another new 1C from 
National Semiconductor: the LM 833 
| dual low-noise opamp. Z1. . .27 are 
1 tuned circuits, details of each of 
which are shown in figure 5 together 
with a table for the individual com- 
ponents for each band. 

The LMC 835 uses CMOS analogue 
switches that have very small leakage 
currents: less than 50 nA. When a 


IMnsass 



MM W 3 


Figure 1. As shown in the block diagram, the LMC 835 contains a digital control 
section and an analogue section consisting of 14 analogue switches. 



;XDO( E>Q00QQ(:3QG)QOQQG)GX; 


Figure 2. The timing diagram of the digital control inputs of the LMC 835. 


1 d8 Boost 

2 dB Boost 

3 dB Boost 

4 dB Boost 

5 dB Boost 

6 dB Boost 

7 dB Boost 

8 dB Boost 

9 dB Boost 

10 d8 Boost 

1 1 dB Boost 

12 dB Boost 

1 dB~l2dBCut 


Th«s is the gam it the 1 12 dB range is 
setocted by DATA I If the ±6 dB 
range is selected, then the values 
shown must be approximately halved 


07 


D5 

D4 

03 

02 

D1 

DO 

L 

X 

L 

L 

L 

L 

L 

L 

L 

H 

H 

L 

L 

L 

l 

L 

L 

H 

L 

H 

L 

L 

L 

L 

L 

H 

L 

L 

H 

L 

L 

L 

L 

H 

L 

L 

L 

H 

L 

L 

L 

H 

L 

L 

L 

L 

H 

L 

L 

H 

L 

H 

L 

L 

H 

L 

L 

H 

H 

L 

H 

L 

H 

l 

L 

H 

L 

H 

L 

H 

H 

L 

l 

H 

L 

L 

L 

L 

L 

H 

L 

H 

H 

L 

H 

l 

L 

H 

L 

H 

H 

L 

H 

H 

L 

H 

L 

H 

H 

L 

H 

H 

H 

H 

L 

L 


Valid Above Input 



1 DATA II 

2 Boost /Cut 


Figure 3. The control data truth tables. The data may be provided by a 
microprocessor. 


3.60 elektor mdia march 1985 







Electrical characteristics 


I 


•“i p-ir-i 
it ii i 

a !!"!! n ! 


IC1* LC11 LC14 «M (MU $TI5Ct | 


m ip m LC< ics ica ict m o mo C16& 


WOM I 

ClOC« t | 

f rt-~i 


Figure 4. The circuit diagram of a 7-band stereo equalizer using the LMC 835. 


DATA I (Band Selection) 


D7 

DC 

D5 

D4 

D3 

D2 

D1 

DO 

H 

X 

L 

L 

L 

L 

L 

L 

H 

X 

L 

L 

L 

L 

L 

H 

H 

X 

L 

L 

L 

L 

H 

L 

H 

X 

L 

L 

L 

L 

H 

H 

H 

X 

L 

L 

l 

H 

L 

L 

H 

X 

L 

L 

L 

H 

L 

H 

H 

X 

L 

L 

L 

H 

H 

L 

H 

X 

L 

l 

L 

H 

H 

H 

H 

X 

L 

L 

H 

L 

L 

L 

H 

X 

L 

L 

H 

L 

L 

H 

H 

X 

L 

L 

H 

L 

H 

L 

H 

X 

L 

L 

H 

L 

H 

H 

H 

X 

l 

L 

H 

H 

L 

L 

H 

X 

l 

L 

H 

H 

l 

H 

H 

X 

L 

L 

H 

H 

H 

L 

H 

X 

L 

L 

H 

H 

H 

H 

H 

X 

L 

H 

Valid Binary Input 

H 

X 

H 

L 

Valid Binary Input 

H 

X 

H 

H 

Valid Binary Input 

t 

t 

t 

T 

4- 

Band Code 


1 

2 

3 

4 






(Ch A: Band 1-7. ChB: Band 8-14) 

Ch A ± 12 dB Range. Ch B t 12 dB Range. No Band Selection 

Ch A * 12 dB Range. Ch B ± 12 dB Range. Band 1 

Ch A X 12 dB Range. Ch B X 12 dB Range. Band 2 

Ch A X 12 dB Range. Ch B X 12 dB Range. Band 3 

Ch A X 12 dB Range. Ch B X 12 dB Range. Band 4 

Ch A X 12 dB Range. Ch B X 12 dB Range. Band 5 

Ch A X 12 dB Range. Ch B X 12 dB Range. Band 6 

Ch A ± 12 dB Range. Ch B X 12 dB Range. Band 7 

Ch A x 12 dB Range. Ch B X 12 dB Range. Band 8 

Ch A X 12 d8 Range. Ch B X 12 dB Range. Band 9 

Ch A x 12 dB Range. Ch B r 12 dB Range, Band 10 

Ch A x 12 dB Range. Ch B ± 12 dB Range. Band 1 1 

Ch A X 12 dB Range. Ch B X 12 dB Range. Band 12 

Ch A X 12 dB Range, Ch B X 12 dB Range. Band 13 

Ch A X 12 dB Range. Ch B X 12 dB Range. Band 14 

Ch A X 12 dB Range. Ch B X 12 dB Range. No Band Selection 

Ch A x 12 dB Range. Ch B x 6 dB Range. Band 1 - 14 

Ch A x 6 dB Range. Ch B i 12 dB Range. Band 1 - 14 

Ch A ± 6 dB Range. Ch 8 X 6 dB Range. Band 1 - 14 


1 DATA 1 

2 Don't Care 

3 Ch A x 6 dB/ X 12 dB Range 

4 Ch B x6 dB/ ± 12 dB Range 


mi ic mi t 3 o* ri 


Q 0 “ 3-5, Qimb= 1.05 


Z1 

to (Hz) 

Co(F) I 

Cl(F) 

*L(tt) 

Ro(n> 

Z1 

63 

lA 

0.1)1 

100k 

680 

Z2 

160 

047 m 

0.033m 

100k 

680 

Z3 

400 

0.1 5m 

0.015)i 

100k 

680 

Z4 

Ik 

0 068 u 

00068m 

82k 

680 

Z5 

2.5k 

0 022 u 

0.0033m 

82k 

680 

Z6 

6.3k 

0.01 Ji 

0.0015m 

62k 

680 

Z7 

16k 

0.0047)x 

680p 

47k 

680 


fed' 


l$-Ci «o 

2wX^o 


| Z7 | 16k | 0.0047m I 680p J 47k j 680 j L — J 

1 (ISMUaSM/lM/m/lk/IkUi 84122-5 

Figure 5. The circuit diagram and c> nponent values for the individual band 
resonators. 



supply voltage 
supply current 
clock frequency 
minimum data 
set-up time 
minimum data 
hold time 
input current 
gain error 
total harmonic 
distortion 

maximum out- 
put voltage 
signal to noise 
ratio 


5. . .16 V 
5 mA maximum 
2 MHz (typical) 


1 JiS 

1 pA maximum 
0.5 dB maximum 

0.1% maximum (at 
1 kHz) 

5 V r.m.s. (minimum) 
106 dB (typical) 



band is selected for flat gain, all the 
switches in that band are open and 
the resonator circuit is not connected 
to the LMC 835 resistor network. It is 
only in the flat mode that the small 
leakage current can cause problems. 
The input to the resonator is a 
capacitor which will be charged 
slowly by the leakage current to a 
high voltage if there is no limiting 
resistor. When the band is set to a 
characteristic other than flat, the 
charge on the capacitor will leak 
away via the resistor network and 
cause a transient at the output. This 
will manifest itself as switching noise 
when the gain is changed. 

To prevent switching noise arising 
from leakage currents, it is necessary 
to include a resistor R|_EAK °f 100 k 
between pin 2 and each of pins 
5. . .11 and between pin 26 and each 
of pins 18. . .24. This resistor, as 
shown in figure 5, limits the voltage 
the capacitor can charge to with 
minimum disturbance to the 
equalization. The consequent gain 
error is only 0.2 dB, while the 
resulting Q error is about 5 per cent 
at 12 dB cut or boost. 

The LMC 835 is expected to become 
available early this year. 


eleklor india march 1985 3.61 







Ml THERMOCOUPLES 

The Ml thermocouples offered by Point 
Electronics consist of a metal sheath 
having the thermoelectric conductors 
embedded with magnesium oxide 
insulation This construction is self 
armoured and protects against 
oxidation or environmental conta- 
mination during service. These 
thermocouples are small in diameter, 
flexible and are impervious to water, 
gas and oil. 

Types K and J are available in 
diameters varying from 1 mm to 4.5 
mm. Amultiple choice of hot and cold 
end terminations are available. 



For further information, write to: 
Point Electronics Pvt. Ltd.. 
1023/1024. IV Block. 

Ftaiaji Nagar. Bangalore 560 010. 


TV COMPONENTS 

Atron Electronics Industries have 
introduced Linearity Coils and Line 
Driver Transformers for 12, 14, and 20 
inch TV sets-Black and White as well as 
Colour. 

The Atron range also covers TV 
components like SMPS Transformers, 
Noise Suppression Filters, Baiun 
Transformers, Degaussing Coils etc. 
The quality and reliability is claimed to 
be very high. 



For further information, write to: 
Atron Electronic Industries 
62— A. Mahatma Gandhi Road, 
Secunderabad 5 00 003. 

3.62 elektor india march 1985 


COMPUTERSCOPE 

The Computerscope— Ind from RC 
Electronics U S A. is a powerful tool 
designed for capturing transient 
signals, with high speed and high 
resolution. The unit is very flexible and 
it can operate as Digital Storage 
Oscilloscope, Digital Voltmeter, Spec- 
trum Analyser, Chart Recorder, Signal 
averager. Histogram Analyser. Fre- 
quency Meter, Multiple Sweep 3— D 
Display and Waveform Analyser The 
acquired data is stored on disc and can 
be displayed conveniently at any time 
for analysis and comparison. A wide 
range of hardware/software options 
are available 



For further information, write to: 
Datacon Systems 
7/8. Vir Bharat, Timber Market. 
Pune 41 1 042. 


CONTROL TRANSFORMERS 

Shepherd Transformers introduce a 
complete range of control transformers 
in single and three phase versions 
These transformers find a wide variety 
of applications in control panels, 
electrical and electronic equipments, 
electromechanical machinery, cranes, 
elevators, material han dli ng equ ipment 
etc 



For further information, write to: 
Shepherd Transformers 
Shed No. 4, Vallabh Society, 

90 leet Road. Ghatkopar (East). 
Bombay 400 075 


POWER INVERTERS 

Jayant Electric and Radio corporation 
offers power inverters with capacities 
from 200 VA to 5 KVA These inverters 
are useful for operation of lights, 
musical instruments, audio and video 
equipment from car batteries The DC 
input can be from 12 to 48 volts 
depending on the mode I and the output 
is 230 V ± 5% sine or square wave at 
50 Hz ± 1%. The circuit is fully solid 
state and meets requirements of JSS 
(Mil, Std.) Efficiencies are claimed to 
be better than 80% for square wave and 
60% for sine wave The construction is 
rugged and can withstand vibrations in 
mobile vehicles even on rough roads 



For further information, write to 
Jayant Electric and Radio Corporation 
5 B. Naigaum Cross Road. 

Wadaia. Bombay 400 031 


TEMPERATURE INDICATOR 

Proteks new Digital Temperature Indi- 
cator uses 25 mm high LEDs fbr the 
digital display for extending the view- 
ing range Measurement ranges avail- 
able are from -200 C to + 1200 C, with 
suitable sensors. Automatic ambient 
temperature compensation and sensor 
break indication are incorporated in 
the circuit The reading stability is 
claimed to be high due to signal being 
routed through amplifier stages. The 
instrument is available in standard DIN 
size and operates directly on mains 



For further information, write to: 

PROTEK 

88/3 Parvati. 

Chintamaninagar. 

Pune 411009 





MEMBRANE KEY BOARD 

Electronumerics have announced a 
new Membrane Key Board The Mem- 
brane Key Board is suitable for various 
instruments and office equipments. It 
can be supplied as standard ASCII 
encoded key board or as custom made 
keyboard complete with attractive 
graphics as per requirement 


CLAP LITE 

t 

Barathtronics introduce a new clap 
controlled bed-light which can be put 
ON/OFF by just a single clap of hands. 
The lamp operates directly on mains 
supply and is sensitive to clapping of 
hands or any similar sound Range of 
operation is about 10 feet Lamp used 
is 6.3 Volts 115 mA bulb, consuming 
approximately 0.6 Watts power. 



For further information, write to: 

Barathtronics 
53, Temple Street. 

Ma lleswaram 
Bangalore 560 003 


CURVE TRACER 

Vasavi Electronics have developed an 
easy to use versatile curve tracer VCT 
12. which can display voltage/current 
characteristics of electronic devices 
like transistors diodes, FETs. etc 
Necessary voltages are provided in the 
instrument for studying the transfer 
characteristics of FETs and triodes 
External oscilloscope having DC coup- 
ling on X and Y system is essential for 
VCT 12 to display the characteristics 


For Further information, write to: 

Electronumerics 
Kammagondanahalli , 

Opp. HMT Industrial Estate. 
Jalahalli (West) 

Bangalore-560 01 5. 



For further information, write to: 
Vasavi Electronics 
6 30. Alkarim Trade Centre 
Ranigunj. Secunderabad 500 003 


POWER OP AMP 

PA 01 is a high voltage, high output 
current operational amplifier designed 
to drive resistive inductive and capaci- 
tive loads. It has a complimentary 
darlington emmitter follower output 
stage protected against inductive kick- 
back or back EMF The Op Amp is 
available in 8 pin TO-3 package which 
is hermetically sealed by one shot 
resistance welding. The PA 01 is 
claimed to be suitable for majority of 
applications in which the uA 791 Op 
Amp is used. 



For Further information, write to : 

Elmatronic Devices 
14 Hanuman Terrace. 

2nd Floor, Lamington Road. 
Bombay 400 007. 


INTERCOM SYSTEM 

CONTACT is the new Intercom System 
introduced by Micro Systems The 
Intercom System covers a wide range 
from 2 lines to 60 lines. It has all the 
standard features such as— LED indi- 
cators. complete privacy, conference 
facility etc. Voice fidelity is claimed to 
be exceptionally high Enclosure is of 
moulded ABS and is available in 
attractive colours. 



For turtner information, write to: 

Micro Systems 
Nilesh Apartments. 

268 Shaniwar Peth. 

Pune 411 030 


ADIVISON 

Advision is a product based on video 
technology for display of information 
on a TV screen. It is a compact, stand 
alone unit with pre programmed 
memory cartridge. The display scrolls 
continuously at preset speed New 
message can be entered by replacing 
the cartridge. Typewriter styled key- 
board is also provided for instant data 
entry. Data can be edited using the 
same keyboard before entering it into 
memory. 

Advision is useful in Banks, Theatres, 
Show Rooms, Hotels, Hospitals, Exhi- 
bitions, Railway Stations, Airports etc. 



For further information, write to: 


Intek Engineers 
7/8 Vir Bharat. 

New Timber Market, 
Pune 411 002 



elektor India inarch t985 3.63 







Make driving 
a pleasure 
with 

car stereo 
speakers 


Luxco car stereo speakers bring 
concert hall performance to you 
— crystal clear Hi-Fi stereo, 
well above the wind 


□ Manufactured by: 

LUXCO Electronics 

Allahabad-211 003 

□ Sole Selling Agents: 

LUXMI & CO. 

56. Johnstonganj 
Allahabad— 211 003. 

Phone: 54041. Telex: 540-486 

□ Distributors for Delhi & Haryana: 
Railton Electronics 

Radio Place. ChandniChowk 
Delhi-110 006. 

Phone: 239944, 233187. 


□ Distributors for Maharashtra. 

Gujarat and South India: 

precious® 

Electronics Corporation 

• Chotani Building, 52. Proctor Road, 
Grant Road (East). Bombay-400 007. 
Phones: 367459. 369478 

• 9, Athipattan. Street. Mount Road. 
Madras-600 002, Phone: 842718 


Wanted stockists all over India 


sound technology from a sound source 


and traffic noise. 




elektor india march 1985 3.67 


Why use just; any Micro O meter 

when 

there are 2 great ones around? 



elektor india march 1 985 3.71 


The Motwane 3 7p Digit L R-204 and 205. 
developed f our inhouse RSD Laboratory are 
Exceptions instruments. To begin with, 
they re a d'gital series enjoying their inherent 
advantages at analog prices Cost/ 
performance bargains in Micro-ohmmeters. 
because of the- excellent accuracy, high 
reliability and effortless operation. 

,The LR series read in 6 ranges each. The 
LR- 204 from 20 milhohms to 2000 ohms 
Cresolution 1 O micro-ohms). TheLR-205 from 
as low as 2 milhohms to 200 ohms 
(resolution 1 micro-ohm). 


Here is the combination of features that make 
these micro-ohmmeters uncommon, 

■ Special circuit to negate those errors 
caused by pick— up in inductive components 
—automatically increasing versatility too. 

■ Pulse mode operation that conveniently 
holds reao ngs and avoids the usual errors 
resulting from heating of the internal circuit / 
samples under measurement. 

■ B.C.D. output for systems capability 

■ Sleek p ast.c casing that provides maximum 
protection and longer lasting good looks, 
with reduced size and weight. 

■ Quality that's exclusive, at a price that's not. 
A system can be built around these instru- 
ments with the following optional accessories: 
® ^ Digita 1 mit comparator for quick go-no-go 

checks. 


SELL ADS.MMC 1 2.84 


P'ense send fiterature and Quotation on your LR Series 

Name 

Designation 

Company 

Address 


■ r-or further details write tc 
THE MOTWANE 
MANUFACTURING COMPANY 

MOTWANE £ VT : l r TD - at G V an Baug. Nasik 
— C Boad 422 1 01 Tel. : 86297/96084 

Telex. 752-247 MMPL IN Grams: MOTWANE 
or Gyan Ghar. Plot 434 A. 14th Road. Khar 
Bombay- 400 052. Grams: MOTESTEM 


■ A D.gital printer for hard copy. 

■ A simple quick mate jig for speedy Q.C. tests. 
When buying a Micro-ohmmeter you really have 
just 2 options. And they are both great! 










R.N. No. 39881/83 


MH/BYW-228 
LIC No 91 



(N 

to 

to 


CD5IT1IC 

COLOURVISION 




l_l_l=3l I 1 1 1_ COLOURVISION 3324 
offers colour with style, 
with it’s built In electronic tuner 

Three decades in the field of electronics has 
helped Cosmic in creating an Audio-Visual marvel 


• 51 cms. The carefully manufactured multifunction unit 

• High intensity picture tube offers vibrant colours 
with perfect sound 

• 8 mode channel selector. 


• Matches any Video system-UHF or VHF 


Off with flying colours 


Printer & Publisher - C- R. Chandarana 2 Kouman. 1 4th A Road. Kher. Bombay -400 052 8. Printed at Trupti Offset. 103. Vasan Udyog Bhavan. 

Off Tulsi Pipe Road. Lower Parel. Bombay 400 01 3. 


Adwel/CTV/345